VclPtr: assert with query dialog open on closing query design

Change-Id: I40d196462ff2e37d86ca367d3a150c94872715f1
(cherry picked from commit 2ecf0ced)

dbaccess/source/ui/querydesign/JoinController.cxx

@@ -170,7 +170,7 @@ OJoinDesignView* OJoinController::getJoinView()
 
 void OJoinController::disposing()
 {
-    m_pAddTableDialog   = NULL;
+    m_pAddTableDialog.disposeAndClear();
 
     OJoinController_BASE::disposing();