Steps to repro:

1. Open Unity on OSX;
2. In the Launcher window sign out of your account;
3. Create new project;
4. In the Editor Click Account - Sign in...;
5. Either click close button or 'Work offline' button.

Actual result:
Launcher window cannot be closed.

Regression introduced in Unity 5.4.0p3.

Workaround is to reopen Editor.

Expected result:
Launcher should always be closed after clicking close button or 'Work Offline' button.

Notes:
- Reproducible in 5.5.0b5, 5.5.0b4, 5.5.0b3, 5.4.1.p2, 5.4.0p3;
- Not reproducible in 5.5.0b2, 5.4.0f3, 5.4.0p2;
- Reproducible only on OSX 10.11.6;
- Not reproducible on Windows 10;
- The issue also reproduces after entering valid login credentials and clicking 'Sign-in' button;
- The issue doesn't reproduce when opening Launcher from Services tab of the Editor.