effects/diminactive: Ensure that m_activeWindow is always reset

As the FIXME comment says, a window can be deleted without prior
windowClosed signal.

BUG: 442222

src/effects/diminactive/diminactive.cpp

@@ -369,6 +369,9 @@ void DimInactiveEffect::windowDeleted(EffectWindow *w)
     // gets destroyed immediately. So, we have to remove active transitions
     // for that window here, otherwise we'll crash in postPaintScreen.
     m_transitions.remove(w);
+    if (m_activeWindow == w) {
+        m_activeWindow = nullptr;
+    }
 }
 
 void DimInactiveEffect::activeFullScreenEffectChanged()