Steps to reproduce:

1. Open Unity on OSX;
2. When Connect window is displayed click right mouse button in OS bottom panel -> Quit.

Actual result:
Fatal error:
'GetManagerFromContext: pointer to object of manager '(null)' is NULL (table index 5)'

is displayed. Window becomes unresponsible. After clicking Quit button in this popup - Connect window crashes.
See attached gif video.

Regression introduced in 5.4.0b1.

Expected result:
Connect window should close without crash after closing it from OSX bottom bar.

Notes:
- Happens only on OSX (10.11.2 checked);
- Doesn't happen on Windows;
- Doesn't happen when closing Connect window by clicking X button inside the window;
- Doesn't happen when closing Editor from OSX bottom panel.