Avoid race condition (e.g. when closing windows)

Avoid race condition (e.g. when closing windows)

The app may become multithreaded e.g. due to the open panel being
displayed.  In this case connectionDidDie notifications may arrive
outside the main thread, possibly leading to windows being closed
simultaneously on multiple threads.  This scenario could happen e.g.
when quitting with multiple windows open.

To avoid this situation performSelectorOnMainThread: is used instead of
performSelector:.