fixed crasher

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@43183 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/aui/framemanager.cpp

@@ -3537,6 +3537,8 @@ void wxAuiManager::UpdateButtonOnScreen(wxAuiDockUIPart* button_ui_part,
                                         const wxMouseEvent& event)
 {
     wxAuiDockUIPart* hit_test = HitTest(event.GetX(), event.GetY());
+    if (!hit_test)
+        return;
         
     int state = wxAUI_BUTTON_STATE_NORMAL;