diff --git a/nw/core/tree.py b/nw/core/tree.py
index ac3ab45c..b76b6678 100644
--- a/nw/core/tree.py
+++ b/nw/core/tree.py
@@ -48,8 +48,10 @@ class NWTree():
         self._treeOrder   = []    # The order of the tree items on the tree view
         self._treeRoots   = []    # The root items of the tree
         self._trashRoot   = None  # The handle of the trash root folder
+        self._archRoot    = None  # The handle of the archive root folder
         self._theIndex    = 0     # The current iterator index
         self._treeChanged = False # True if tree structure has changed
+
         self._handleSeed  = None  # Used for generating handles for testing
 
         return
@@ -83,14 +85,15 @@ class NWTree():
         if tHandle is None:
             tHandle = self._makeHandle()
 
+        if tHandle in self._projTree:
+            logger.warning("Duplicate handle %s detected, generating new" % tHandle)
+            tHandle = self._makeHandle()
+
         logger.verbose("Adding item %s with parent %s" % (str(tHandle), str(pHandle)))
 
         nwItem.setHandle(tHandle)
         nwItem.setParent(pHandle)
 
-        self._projTree[tHandle] = nwItem
-        self._treeOrder.append(tHandle)
-
         if nwItem.itemType == nwItemType.ROOT:
             logger.verbose("Item %s is a root item" % str(tHandle))
             self._treeRoots.append(tHandle)
@@ -104,7 +107,10 @@ class NWTree():
                 self._trashRoot = tHandle
             else:
                 logger.error("Only one trash folder allowed")
+                return
 
+        self._projTree[tHandle] = nwItem
+        self._treeOrder.append(tHandle)
         self._setTreeChanged(True)
 
         return
@@ -176,8 +182,9 @@ class NWTree():
 
         except Exception as e:
             logger.error(str(e))
+            return False
 
-        return
+        return True
 
     def sumWords(self):
         """Loops over all entries and adds up the word counts.
@@ -244,6 +251,8 @@ class NWTree():
             return True
         for aRoot in self._treeRoots:
             tItem = self.__getitem__(aRoot)
+            if tItem is None:
+                continue
             if theClass == tItem.itemClass:
                 return False
         return True
@@ -397,7 +406,7 @@ class NWTree():
             del self._projTree[tHandle]
         else:
             logger.warning("Failed to delete item %s: item not found" % tHandle)
-            return False
+            return
 
         if tHandle in self._treeRoots:
             self._treeRoots.remove(tHandle)
@@ -408,7 +417,7 @@ class NWTree():
 
         self._setTreeChanged(True)
 
-        return True
+        return
 
     def __contains__(self, tHandle):
         """Checks if a handle exists in the tree.
diff --git a/tests/test_core_tree.py b/tests/test_core_tree.py
new file mode 100644
index 00000000..1eff405d
--- /dev/null
+++ b/tests/test_core_tree.py
@@ -0,0 +1,437 @@
+# -*- coding: utf-8 -*-
+"""novelWriter NWTree Class Tester
+"""
+
+import os
+import pytest
+
+from lxml import etree
+
+from nw.core.project import NWProject, NWItem, NWTree
+from nw.constants import nwItemClass, nwItemType, nwItemLayout, nwFiles
+
+@pytest.fixture(scope="session")
+def dummyItems(dummyGUI):
+    """Create a list of dummy items.
+    """
+    theProject = NWProject(dummyGUI)
+
+    itemA = NWItem(theProject)
+    itemA.itemName = "Novel"
+    itemA.itemType = nwItemType.ROOT
+    itemA.itemClass = nwItemClass.NOVEL
+    itemA.isExpanded = True
+
+    itemB = NWItem(theProject)
+    itemB.itemName = "Act One"
+    itemB.itemType = nwItemType.FOLDER
+    itemB.itemClass = nwItemClass.NOVEL
+    itemB.isExpanded = True
+
+    itemC = NWItem(theProject)
+    itemC.itemName = "Chapter One"
+    itemC.itemType = nwItemType.FILE
+    itemC.itemClass = nwItemClass.NOVEL
+    itemC.itemLayout = nwItemLayout.CHAPTER
+    itemC.charCount = 300
+    itemC.wordCount = 50
+    itemC.paraCount = 2
+
+    itemD = NWItem(theProject)
+    itemD.itemName = "Scene One"
+    itemD.itemType = nwItemType.FILE
+    itemD.itemClass = nwItemClass.NOVEL
+    itemD.itemLayout = nwItemLayout.SCENE
+    itemD.charCount = 3000
+    itemD.wordCount = 500
+    itemD.paraCount = 20
+
+    itemE = NWItem(theProject)
+    itemE.itemName = "Outtakes"
+    itemE.itemType = nwItemType.ROOT
+    itemE.itemClass = nwItemClass.ARCHIVE
+    itemE.isExpanded = False
+
+    itemF = NWItem(theProject)
+    itemF.itemName = "Trash"
+    itemF.itemType = nwItemType.TRASH
+    itemF.itemClass = nwItemClass.TRASH
+    itemF.isExpanded = False
+
+    itemG = NWItem(theProject)
+    itemG.itemName = "Characters"
+    itemG.itemType = nwItemType.ROOT
+    itemG.itemClass = nwItemClass.CHARACTER
+    itemG.isExpanded = True
+
+    itemH = NWItem(theProject)
+    itemH.itemName = "Jane Doe"
+    itemH.itemType = nwItemType.FILE
+    itemH.itemClass = nwItemClass.CHARACTER
+    itemH.itemLayout = nwItemLayout.NOTE
+    itemH.charCount = 2000
+    itemH.wordCount = 400
+    itemH.paraCount = 16
+
+    theItems = [
+        ("a000000000001", None,            itemA),
+        ("b000000000001", "a000000000001", itemB),
+        ("c000000000001", "b000000000001", itemC),
+        ("c000000000002", "b000000000001", itemD),
+        ("a000000000002", None,            itemE),
+        ("a000000000003", None,            itemF),
+        ("a000000000004", None,            itemG),
+        ("b000000000002", "a000000000002", itemH),
+    ]
+
+    return theItems
+
+@pytest.mark.core
+def testCoreTree_BuildTree(dummyGUI, dummyItems):
+    """Test building a project tree from a list of items.
+    """
+    theProject = NWProject(dummyGUI)
+    theTree = NWTree(theProject)
+
+    theTree.setSeed(42)
+    assert theTree._handleSeed == 42
+
+    # Check that tree is empty (calls NWTree.__bool__)
+    assert not theTree
+
+    # Check for archive and trash folders
+    assert theTree.trashRoot() is None
+    assert theTree.archiveRoot() is None
+    assert not theTree.isTrashRoot("a000000000003")
+
+    aHandles = []
+    for tHandle, pHande, nwItem in dummyItems:
+        aHandles.append(tHandle)
+        theTree.append(tHandle, pHande, nwItem)
+
+    assert theTree._treeChanged
+
+    # Check that tree is not empty (calls __bool__)
+    assert theTree
+
+    # Check the number of elements (calls __len__)
+    assert len(theTree) == len(dummyItems)
+
+    # Check that we have the correct handles
+    assert theTree.handles() == aHandles
+
+    # Check by iterator (calls __iter__, __next__ and __getitem__)
+    for theItem, theHandle in zip(theTree, aHandles):
+        assert theItem.itemHandle == theHandle
+
+    # Check that we have the correct archive and trash folders
+    assert theTree.trashRoot() == "a000000000003"
+    assert theTree.archiveRoot() == "a000000000002"
+    assert theTree.isTrashRoot("a000000000003")
+
+    # Try to add another trash folder
+    itemT = NWItem(theProject)
+    itemT.itemName = "Trash"
+    itemT.itemType = nwItemType.TRASH
+    itemT.itemClass = nwItemClass.TRASH
+    itemT.isExpanded = False
+
+    theTree.append("1234567890abc", None, itemT)
+    assert len(theTree) == len(dummyItems)
+
+    # Generate handle automatically
+    itemT = NWItem(theProject)
+    itemT.itemName = "New File"
+    itemT.itemType = nwItemType.FILE
+    itemT.itemClass = nwItemClass.NOVEL
+    itemT.itemLayout = nwItemLayout.SCENE
+
+    theTree.append(None, None, itemT)
+    assert len(theTree) == len(dummyItems) + 1
+
+    theList = theTree.handles()
+    assert theList[-1] == "73475cb40a568"
+
+    # Duplicate handle
+    itemT = NWItem(theProject)
+    itemT.itemName = "New File"
+    itemT.itemType = nwItemType.FILE
+    itemT.itemClass = nwItemClass.NOVEL
+    itemT.itemLayout = nwItemLayout.SCENE
+
+    theTree.append("73475cb40a568", None, itemT)
+    assert len(theTree) == len(dummyItems) + 2
+
+    theList = theTree.handles()
+    assert theList[-1] == "44cb730c42048"
+
+    # Delete the last two items
+    del theTree["dummy"]
+    assert len(theTree) == len(dummyItems) + 2
+
+    del theTree["44cb730c42048"]
+    assert len(theTree) == len(dummyItems) + 1
+    assert "44cb730c42048" not in theTree
+
+    del theTree["73475cb40a568"]
+    assert len(theTree) == len(dummyItems)
+    assert "73475cb40a568" not in theTree
+
+    del theTree["a000000000001"]
+    assert len(theTree) == len(dummyItems) - 1
+    assert "a000000000001" not in theTree
+
+    del theTree["a000000000002"]
+    assert len(theTree) == len(dummyItems) - 2
+    assert "a000000000002" not in theTree
+    assert theTree.archiveRoot() is None
+
+    del theTree["a000000000003"]
+    assert len(theTree) == len(dummyItems) - 3
+    assert "a000000000003" not in theTree
+    assert theTree.trashRoot() is None
+
+# END Test testCoreTree_BuildTree
+
+@pytest.mark.core
+def testCoreTree_Methods(dummyGUI, dummyItems):
+    """Test building a project tree from a list of items.
+    """
+    theProject = NWProject(dummyGUI)
+    theTree = NWTree(theProject)
+
+    for tHandle, pHande, nwItem in dummyItems:
+        theTree.append(tHandle, pHande, nwItem)
+
+    assert len(theTree) == len(dummyItems)
+
+    # Root item lookup
+    theTree._treeRoots.append("dummy")
+    assert theTree.findRoot(nwItemClass.WORLD) is None
+    assert theTree.findRoot(nwItemClass.NOVEL) == "a000000000001"
+    assert theTree.findRoot(nwItemClass.CHARACTER) == "a000000000004"
+
+    # Check for root uniqueness
+    assert theTree.checkRootUnique(nwItemClass.CUSTOM)
+    assert theTree.checkRootUnique(nwItemClass.WORLD)
+    assert not theTree.checkRootUnique(nwItemClass.NOVEL)
+    assert not theTree.checkRootUnique(nwItemClass.CHARACTER)
+
+    # Find root item of child item
+    assert theTree.getRootItem("b000000000001").itemHandle == "a000000000001"
+    assert theTree.getRootItem("c000000000001").itemHandle == "a000000000001"
+    assert theTree.getRootItem("c000000000002").itemHandle == "a000000000001"
+    assert theTree.getRootItem("dummy") is None
+
+    # Get item path
+    assert theTree.getItemPath("dummy") == []
+    assert theTree.getItemPath("c000000000001") == [
+        "c000000000001", "b000000000001", "a000000000001"
+    ]
+
+    # Break the folder parent handle
+    theTree["b000000000001"].itemParent = "dummy"
+    assert theTree.getItemPath("c000000000001") == [
+        "c000000000001", "b000000000001"
+    ]
+
+    theTree["b000000000001"].itemParent = "a000000000001"
+    assert theTree.getItemPath("c000000000001") == [
+        "c000000000001", "b000000000001", "a000000000001"
+    ]
+
+    # Change file layout
+    assert not theTree.setFileItemLayout("dummy", nwItemLayout.UNNUMBERED)
+    assert not theTree.setFileItemLayout("b000000000001", nwItemLayout.UNNUMBERED)
+    assert not theTree.setFileItemLayout("c000000000001", "stuff")
+    assert theTree.setFileItemLayout("c000000000001", nwItemLayout.UNNUMBERED)
+    assert theTree["c000000000001"].itemLayout == nwItemLayout.UNNUMBERED
+
+# END Test testCoreTree_Methods
+
+@pytest.mark.core
+def testCoreTree_MakeHandles(dummyGUI, monkeypatch):
+    """Test generating item handles.
+    """
+    theProject = NWProject(dummyGUI)
+    theTree = NWTree(theProject)
+
+    theTree.setSeed(42)
+
+    tHandle = theTree._makeHandle()
+    assert tHandle == "73475cb40a568"
+
+    # Add the next in line to the project to foprce duplicate
+    theTree._projTree["44cb730c42048"] = None
+    tHandle = theTree._makeHandle()
+    assert tHandle == "71ee45a3c0db9"
+
+    # Fix the time() function and force a handle collission
+    theTree.setSeed(None)
+    monkeypatch.setattr("nw.core.tree.time", lambda: 123.4)
+
+    tHandle = theTree._makeHandle()
+    theTree._projTree[tHandle] = None
+    assert tHandle == "5f466d7afa48b"
+
+    tHandle = theTree._makeHandle()
+    theTree._projTree[tHandle] = None
+    assert tHandle == "a79acf4c634a7"
+
+    monkeypatch.undo()
+
+# END Test testCoreTree_MakeHandles
+
+@pytest.mark.core
+def testCoreTree_Stats(dummyGUI, dummyItems):
+    """Test project stats methods.
+    """
+    theProject = NWProject(dummyGUI)
+    theTree = NWTree(theProject)
+
+    for tHandle, pHande, nwItem in dummyItems:
+        theTree.append(tHandle, pHande, nwItem)
+
+    assert len(theTree) == len(dummyItems)
+    theTree._treeOrder.append("dummy")
+
+    # Count Words
+    novelWords, noteWords = theTree.sumWords()
+    assert novelWords == 550
+    assert noteWords == 400
+
+    # Count types
+    nRoot, nFolder, nFile = theTree.countTypes()
+    assert nRoot == 3
+    assert nFolder == 1
+    assert nFile == 3
+
+# END Test testCoreTree_Stats
+
+@pytest.mark.core
+def testCoreTree_Reorder(dummyGUI, dummyItems):
+    """Test changing tree order.
+    """
+    theProject = NWProject(dummyGUI)
+    theTree = NWTree(theProject)
+
+    aHandle = []
+    for tHandle, pHande, nwItem in dummyItems:
+        aHandle.append(tHandle)
+        theTree.append(tHandle, pHande, nwItem)
+
+    assert len(theTree) == len(dummyItems)
+
+    bHandle = aHandle.copy()
+    bHandle[2], bHandle[3] = bHandle[3], bHandle[2]
+    assert aHandle != bHandle
+
+    assert theTree.handles() == aHandle
+    theTree.setOrder(bHandle)
+    assert theTree.handles() == bHandle
+
+    theTree.setOrder(bHandle + ["dummy"])
+    assert theTree.handles() == bHandle
+
+    theTree._treeOrder.append("dummy")
+    theTree.setOrder(bHandle)
+    assert theTree.handles() == bHandle
+
+# END Test testCoreTree_Reorder
+
+@pytest.mark.core
+def testCoreTree_XMLPackUnpack(dummyGUI, dummyItems):
+    """Test changing tree order.
+    """
+    theProject = NWProject(dummyGUI)
+    theTree = NWTree(theProject)
+
+    for tHandle, pHande, nwItem in dummyItems:
+        theTree.append(tHandle, pHande, nwItem)
+
+    assert len(theTree) == len(dummyItems)
+
+    nwXML = etree.Element("novelWriterXML")
+    theTree.packXML(nwXML)
+    assert etree.tostring(nwXML, pretty_print=False, encoding="utf-8") == (
+        b"<novelWriterXML>"
+        b"<content count=\"8\">"
+        b"<item handle=\"a000000000001\" order=\"None\" parent=\"None\">"
+        b"<name>Novel</name><type>ROOT</type><class>NOVEL</class><status>None</status>"
+        b"<expanded>True</expanded></item>"
+        b"<item handle=\"b000000000001\" order=\"None\" parent=\"a000000000001\">"
+        b"<name>Act One</name><type>FOLDER</type><class>NOVEL</class><status>None</status>"
+        b"<expanded>True</expanded></item>"
+        b"<item handle=\"c000000000001\" order=\"None\" parent=\"b000000000001\">"
+        b"<name>Chapter One</name><type>FILE</type><class>NOVEL</class><status>None</status>"
+        b"<exported>True</exported><layout>UNNUMBERED</layout><charCount>300</charCount>"
+        b"<wordCount>50</wordCount><paraCount>2</paraCount><cursorPos>0</cursorPos></item>"
+        b"<item handle=\"c000000000002\" order=\"None\" parent=\"b000000000001\">"
+        b"<name>Scene One</name><type>FILE</type><class>NOVEL</class><status>None</status>"
+        b"<exported>True</exported><layout>SCENE</layout><charCount>3000</charCount>"
+        b"<wordCount>500</wordCount><paraCount>20</paraCount><cursorPos>0</cursorPos></item>"
+        b"<item handle=\"a000000000002\" order=\"None\" parent=\"None\">"
+        b"<name>Outtakes</name><type>ROOT</type><class>ARCHIVE</class><status>None</status>"
+        b"<expanded>False</expanded></item>"
+        b"<item handle=\"a000000000003\" order=\"None\" parent=\"None\">"
+        b"<name>Trash</name><type>TRASH</type><class>TRASH</class><status>None</status>"
+        b"<expanded>False</expanded></item>"
+        b"<item handle=\"a000000000004\" order=\"None\" parent=\"None\">"
+        b"<name>Characters</name><type>ROOT</type><class>CHARACTER</class><status>None</status>"
+        b"<expanded>True</expanded></item>"
+        b"<item handle=\"b000000000002\" order=\"None\" parent=\"a000000000002\">"
+        b"<name>Jane Doe</name><type>FILE</type><class>CHARACTER</class><status>None</status>"
+        b"<exported>True</exported><layout>NOTE</layout><charCount>2000</charCount>"
+        b"<wordCount>400</wordCount><paraCount>16</paraCount><cursorPos>0</cursorPos></item>"
+        b"</content></novelWriterXML>"
+    )
+
+    theTree.clear()
+    assert len(theTree) == 0
+    assert not theTree.unpackXML(nwXML)
+    assert theTree.unpackXML(nwXML[0])
+    assert len(theTree) == len(dummyItems)
+
+# END Test testCoreTree_XMLPackUnpack
+
+@pytest.mark.core
+def testCoreTree_ToCFile(dummyGUI, dummyItems, tmpDir, monkeypatch):
+    """Test writing the ToC.txt file.
+    """
+    theProject = NWProject(dummyGUI)
+    theTree = NWTree(theProject)
+
+    for tHandle, pHande, nwItem in dummyItems:
+        theTree.append(tHandle, pHande, nwItem)
+
+    assert len(theTree) == len(dummyItems)
+    theTree._treeOrder.append("dummy")
+
+    monkeypatch.setattr("os.path.isfile", lambda *args: True)
+
+    theProject.projContent = "content"
+    theProject.projPath = None
+    assert not theTree.writeToCFile()
+
+    theProject.projPath = tmpDir
+    assert theTree.writeToCFile()
+
+    with open(os.path.join(tmpDir, nwFiles.TOC_TXT), mode="r", encoding="utf8") as inFile:
+        assert inFile.read() == (
+            "\n"
+            "Table of Contents\n"
+            "=================\n"
+            "\n"
+            "File Name                  Class      Layout      Document Label\n"
+            "-------------------------------------------------------------\n"
+            "content/a000000000001.nwd  NOVEL      NO_LAYOUT   Novel\n"
+            "content/b000000000001.nwd  NOVEL      NO_LAYOUT   Act One\n"
+            "content/c000000000001.nwd  NOVEL      UNNUMBERED  Chapter One\n"
+            "content/c000000000002.nwd  NOVEL      SCENE       Scene One\n"
+            "content/a000000000002.nwd  ARCHIVE    NO_LAYOUT   Outtakes\n"
+            "content/a000000000003.nwd  TRASH      NO_LAYOUT   Trash\n"
+            "content/a000000000004.nwd  CHARACTER  NO_LAYOUT   Characters\n"
+            "content/b000000000002.nwd  CHARACTER  NOTE        Jane Doe\n"
+        )
+
+# END Test testCoreTree_ToCFile