To reproduce:

1. Download attached project "proto-melee.zip" and open in Unity
2. Open "Main" Scene
3. Enter Play mode
4. Click the left button of the mouse

Expected Result: After a delay, a character should swing their weapon
Actual Result: After a delay, a character swings their weapon. Then the editor will crash.

Reproduced on Unity 5.5.4f1, 5.6.1p4, 2017.1.0b9 and 2017.2.0a3

Note: This issue appears on both Windows and OSX