VclPtr: no dialog loaded from a .ui ever actually destructed

Change-Id: I51a0596049a43cbc80f914f3d8491c2125c8109e
Signed-off-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/18158Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/window/builder.cxx

@@ -543,6 +543,7 @@ void VclBuilder::disposeBuilder()
         delete aI->m_pMenu;
     }
     m_aMenus.clear();
+    m_pParent.clear();
 }
 
 void VclBuilder::handleTranslations(xmlreader::XmlReader &reader)