Sanitize special window pointers in flip/coverswitch

CCBUG:253870

effects/coverswitch/coverswitch.cpp

@@ -988,6 +988,8 @@ void CoverSwitchEffect::abort()
 
 void CoverSwitchEffect::slotWindowClosed(EffectWindow* c)
 {
+    if (c == selected_window)
+        selected_window = 0;
     // if the list is not empty, the effect is active
     if (!currentWindowList.isEmpty()) {
         c->refWindow();

effects/flipswitch/flipswitch.cpp

@@ -591,6 +591,8 @@ void FlipSwitchEffect::slotWindowAdded(EffectWindow* w)
 
 void FlipSwitchEffect::slotWindowClosed(EffectWindow* w)
 {
+    if (m_selectedWindow == w)
+        m_selectedWindow = 0;
     if (m_active && m_windows.contains(w)) {
         m_windows.remove(w);
     }