To reproduce:

1. Open attached project;
2. Open "TestBug" scene;
3. Enter play mode;
4. Exit play mode.

Expected result: Editor doesn't freeze when console throws KeyNotFoundException.
Actual result: when KeyNotFoundException appears in console, the game crashes, editor freezes.

Reproduced on versions: 5.4.0p3, 5.5.0f3, 5.6.0b4.
Failed to load on 5.3 versions, console throws errors:
"Failed to load 'Assets/TestBug.unity' because its serialized file version is higher than what this version of Unity supports."(5.3.0f4, 5.3.4f1);
"CheckDisalowAllocation. Allocating memory when it is not allowed to allocate memory" (on 5.3.7p3 unity freezes when trying to quit).