Steps to repro:

1. Open Unity Launcher Window on OSX;
2. Click Sign In button;
3. Close Launcher window (either by clicking X button or by clicking right button in OSX Dock -> Quit);
4. Open Launcher window again;
5. Either input correct e-mail and password and click 'Sign In' button or click 'Work Offline'.

Actual result:
Launcher crashes (quits). See attached video.
Regression introduced in Unity 5.3.

Expected result:
Launcher should not crash after inputting correct login credentials; List of recently opened projects should be displayed in this case.

Workaround is to reopen Launcher window again.

Notes:
- Reproducible in: 5.4.0b11, 5.4.0b10, 5.4.0b8, 5.3.4f1, 5.3.3f1, 5.3.2f1;
- Not reproducible in 5.2.2p4;
- Reproducible only on OSX (10.11.3 checked);
- Not reproducible on Windows 10;
- In case if the issue doesn't repro, try following:
1) Logout from your Unity account;
2) Create new project in Editor;
3) Close Editor;
4) Open Launcher;
5) Repeat all the initial repro steps mentioned above.