To reproduce:
1. Create and open new project
2. Open services tab (command+ 0)
3. Close services tab following these instructions:
1) if services tab is in main window, hold control key and release it a few milliseconds later after clicking 'close tab'(this method works only sometimes, does not work on 5.5.0b10);
2) if services tab is in seperate window, hold control key when closing tab
4. Repeat 2 and 3 these steps until Unity will crash

Reproduced on 5.5.0b7, 5.5.0b8, 5.5.0b10
Unable to reproduce on 5.4.2p1, 5.4.1p4