Steps to reproduce:

1. Open Unity;
2. Open attached project and 'Main.unity' scene;
3. Go to File --> Build Settings;
4. Click Player Settings button;
5. Enter Play mode;
6. In Player Settings' Optimization tab switch Api compatibility level (from .Net 2.0 Subset to .Net 2.0 or vise versa);
7. Quickly exit Play mode.

Actual result:
Unity crashes (see attached gif video).
Regression introduced in Unity 5.4.0.

Expected result:
Unity should not crash after switching API compatibility level at runtime.

Notes:
- Reproducible on 5.4.0b1, 5.4.0a6, 5.4.0a5, 5.4.0a2;
- Not reproducible in 5.3.0f4;
- Reproduced on Windows 10;
- Crash may not happen each time, to increase reproduction probability you should leave Play mode after switching compatibility modes quickly.