layersurface: reset popuphead after unmap

fixes #5980

src/desktop/LayerSurface.cpp

@@ -73,8 +73,6 @@ void CLayerSurface::onDestroy() {
 
     const auto PMONITOR = g_pCompositor->getMonitorFromID(monitorID);
 
-    popupHead.reset();
-
     if (!g_pCompositor->getMonitorFromID(monitorID))
         Debug::log(WARN, "Layersurface destroyed on an invalid monitor (removed?)");
 
@@ -90,6 +88,8 @@ void CLayerSurface::onDestroy() {
         }
     }
 
+    popupHead.reset();
+
     noProcess = true;
 
     // rearrange to fix the reserved areas