showbase: Fix DistancePhasedNode exception at module clean-up time

direct/src/showbase/DistancePhasedNode.py

@@ -78,7 +78,8 @@ class DistancePhasedNode(PhasedObject, DirectObject, NodePath):
         """
         Reuse abandoned ids.
         """
-        DistancePhasedNode.__InstanceDeque.append(id)
+        if DistancePhasedNode is not None:
+            DistancePhasedNode.__InstanceDeque.append(id)
 
     def __init__(self, name, phaseParamMap = {},
                  autoCleanup = True,