Fix error on Windows after deleting fit node.

Fit deletion triggers onTreeSelChanged handler with event holding
the deleted and memory-released phase node.  The handler calls
GetNodeType, but all node data are already gone.

Solution: Return None from GetNodeType if GetTreeItemDict gives None.

Note: Might be better to somehow disable events from the deleted nodes.

Note: Shows only on Windows, dead-node events are not raised on Mac.

Author: pavoljuhas

diffpy/pdfgui/gui/fittree.py

@@ -176,17 +176,21 @@ def GetNodeType(self, node):
 
         This is the "type" entry in the data dictionary of the node.
         """
-        if not node: return
-        return self.GetTreeItemDict(node)['type']
+        if not node:
+            return None
+        datadict = self.GetTreeItemDict(node)
+        if datadict is None:
+            return None
+        return datadict['type']
 
-    def SetNodeType(self, node, type):
+    def SetNodeType(self, node, tp):
         """Set the node type of a node."""
         if not node: return
         datadict = self.GetTreeItemDict(node)
-        if not datadict:
-            self.SetPyData(node, {'type': type})
-        else:
-            self.GetTreeItemDict(node)['type'] = type
+        if datadict is None:
+            datadict = {}
+            self.SetPyData(node, datadict)
+        datadict['type'] = tp
         return
 
     def GetBranchName(self, node):