[macOS] Crash in libqcocoa.dylib when disconnecting external monitors

The crash happens in the cocoa plugin. The crash came from users but I managed to reproduce it:

• MacBook Pro 2015 with 2 external monitors connected, using wall power (not to sleep when lid is closed), our Qt app is running in the background
• Now close the lid -> we have now 2 external monitors only (no internal screen)
• Now pull both miniDisplayPort cables out of the macbook and connect again after couple of seconds -> the Qt app crashed