Fix crash when restoring state

- when restoring state, the old dock widgets would remove themselves
from their old dock areas and add themselves to the new dock areas. The
old dock areas that are empty would then delete themselves.
- That is a problem when the old overlay dock container would still try to
access the old dock areas, not knowing their deleted, causing a crash.

src/DockContainerWidget.cpp

@@ -1666,6 +1666,7 @@ void CDockContainerWidget::removeDockArea(CDockAreaWidget* area)
         CDockWidget::emitTopLevelEventForWidget(TopLevelWidget, true);
         dumpLayout();
         d->emitDockAreasRemoved();
+		area->overlayDockContainer()->clearDockArea();
         area->setOverlayDockContainer(nullptr);
 		area->updateAutoHideButtonCheckState();
         area->updateTitleBarButtonToolTip();