wxGUI/mapswipe: call dialog Uninit only when dialog was actually created, fix for #833 (#836)

gui/wxpython/mapswipe/frame.py

@@ -754,7 +754,8 @@ class SwipeMapFrame(DoubleMapFrame):
         self.GetFirstMap().Clean()
         self.GetSecondMap().Clean()
         self._mgr.UnInit()
-        self._inputDialog.UnInit()
+        if self._inputDialog:
+            self._inputDialog.UnInit()
         self.Destroy()