core: Add null activeWorkspace check in moveWorkspaceToMonitor (#9503)

src/Compositor.cpp

@@ -2210,6 +2210,7 @@ void CCompositor::moveWorkspaceToMonitor(PHLWORKSPACE pWorkspace, PHLMONITOR pMo
     // finalize
     if (POLDMON) {
         g_pLayoutManager->getCurrentLayout()->recalculateMonitor(POLDMON->ID);
+        if (valid(POLDMON->activeWorkspace))
             updateFullscreenFadeOnWorkspace(POLDMON->activeWorkspace);
         updateSuspendedStates();
     }