Explicity get set manager as None if appropiate.

OceanWolf · OceanWolf · commit 8a4268ab9b23 · 2016-08-04T01:37:31.000+02:00
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -2497,11 +2497,14 @@ def destroy(self):
     def manager(self):
         if self._manager is not None:
             return self._manager()
+        return None
 
     @manager.setter
     def manager(self, manager):
         if manager is not None:
             self._manager = weakref.ref(manager)
+        else:
+            self._manager = None
 
 
 def key_press_handler(event, canvas, toolbar=None):
