Restore focus from cell par "a" only when possible.

On Windows the previous focus owner is None therefore the
focus cannot be restored.

Author: pavoljuhas

diffpy/pdfgui/gui/phaseconfigurepanel.py

@@ -266,7 +266,8 @@ def refresh(self):
             self.__this_is_first_refresh = False
             focusowner = self.textCtrlA.FindFocus()
             wx.CallAfter(self.textCtrlA.SetFocus)
-            wx.CallAfter(focusowner.SetFocus)
+            if focusowner is not None:
+                wx.CallAfter(focusowner.SetFocus)
         return
 
     def restrictConstrainedParameters(self):