Merge pull request enthought#92 from pankajp/pymimedata-coerce-no-ins…

…tance

Avoid qt traceback on drag/drop/clipboard operations.

traitsui/qt4/clipboard.py

@@ -83,6 +83,10 @@ def instance(self):
         if self._local_instance is not None:
             return self._local_instance
 
+        if not self.hasFormat(self.MIME_TYPE):
+            # We have no pickled python data defined.
+            return None
+
         io = StringIO(str(self.data(self.MIME_TYPE)))
 
         try:

traitsui/qt4/tree_editor.py

@@ -513,8 +513,9 @@ def _delete_node ( self, nid ):
             pnid.removeChild(nid)
 
         # If the deleted node had an active editor panel showing, remove it:
-        if (self._editor is not None) and (nid == self._editor._editor_nid):
-            self._clear_editor()
+        # Note: QTreeWidgetItem does not have an equal operator, so use id()
+        if (self._editor is not None) and (id(nid) == id(self._editor._editor_nid)):
+                self._clear_editor()
 
     #---------------------------------------------------------------------------
     #  Expands the contents of a specified node (if required):
@@ -1785,10 +1786,12 @@ def dragMoveEvent(self, e):
         # A copy action is interpreted as moving the source to a particular
         # place within the target's parent.  A move action is interpreted as
         # moving the source to be a child of the target.
+        node = None
         if e.proposedAction() == QtCore.Qt.CopyAction:
             node, object, _ = self._editor._node_index(nid)
+            # `node` is None in case of top-level tree node, in which case we try move
             insert = True
-        else:
+        if node is None:
             _, node, object = self._editor._get_node_data(nid)
             insert = False