Fixed some potential issues with getting the trash folder handle, and made some minor improvements to index class methods

nw/core/tree.py

@@ -213,6 +213,13 @@ class NWTree():
             return self._trashRoot
         return None
 
+    def isTrashRoot(self, tHandle):
+        """Check if a handle is the trash folder.
+        """
+        if self._trashRoot is None:
+            return False
+        return tHandle == self._trashRoot
+
     def archiveRoot(self):
         """Returns the handle of the archive folder, or None if there
         isn't one.