Correct weak ref usage which triggered an assert

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@53164 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/datavcmn.cpp

@@ -678,6 +678,7 @@ public:
    virtual void OnObjectDestroy()
    {
       get()->PopEventHandler( true );
+      m_pobj = NULL;
       delete this;
    }
 };