Fix exception at deleting the second Fit.

During deletion of the second Fit the onTreeSelChanged handler
got triggered by the last dataset from fit1, but GetSelections()
list gave fit2.  This caused the GUI to use the Fitting object
(selections) for updating dataset panel (from event).

Solution: Ignore type of the node that triggered onTreeSelChanged.
Only use the GetSelections() nodes for updating right-side panel.

Author: pavoljuhas

diffpy/pdfgui/gui/mainframe.py

@@ -1103,7 +1103,7 @@ def treeSelectionUpdate(self, node):
                 #return
             elif len(selections) == 1:
                 self.rightPanel.Enable()
-                selectiontype = self.treeCtrlMain.GetNodeType(node)
+                selectiontype = self.treeCtrlMain.GetNodeType(selections[0])
                 self.switchRightPanel(selectiontype)
             else:
                 self.rightPanel.Enable(False)