ignore disposed toplevels

Change-Id: Ib5a9bad2537d43f7b6588dbdb958e4cdd34f7023
Reviewed-on: https://gerrit.libreoffice.org/68385
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/source/window/dialog.cxx

@@ -1569,7 +1569,11 @@ void TopLevelWindowLocker::decBusy()
 {
     // unlock locked toplevel windows from being closed now busy is over
     for (auto& a : m_aBusyStack.top())
+    {
+        if (a->IsDisposed())
+            continue;
         a->DecModalCount();
+    }
     m_aBusyStack.pop();
 }