layerSurface: check for monitor validity in startAnimation

ref #11168

sometimes on exit monitor might be null

src/desktop/LayerSurface.cpp

@@ -470,6 +470,11 @@ void CLayerSurface::startAnimation(bool in, bool instant) {
 
         const auto PMONITOR = g_pCompositor->getMonitorFromVector(MIDDLE);
 
+        if (!PMONITOR) { // can rarely happen on exit
+            m_alpha->setValueAndWarp(in ? 1.F : 0.F);
+            return;
+        }
+
         int      force = -1;
 
         CVarList args(ANIMSTYLE, 0, 's');