How to reproduce:
1. Open user-submitted project (CS 110 Game Remake Remake.zip)
2. Open "Level01.unity" scene
3. Play the scene
4. Position 3 towers on the map
5. Keep clicking start button(In-game) for the entire game(Best to use auto-clicking software; Crash usually happens at the beginning of Wave 7)

Expected result: Unity does not crash
Actual result: Unity crashes while spawning another wave while the last one has not been killed

Reproducible with: 2017.3.0b1, 2017.4.5f1, 2018.1.4f1, 2018.2.0b7, 2018.3.0a1
Not reproducible with: 2017.1.4p1, 2017.2.3p1, 2017.3.0a1, 2017.3.0a7
Could not test with:

Notes:
* Cleaning unnecessary project data doesn't help
* Builds reproduce the issue better than Editor(tested on Windows PC)
* Trying to instantiate SlimeFull prefabs as fast as possible in a new project does not reproduce the issue
* Could be related to:
+ Coroutines - enemy spawn method is executed in a coroutine
+ UI - SlimeFull prefab has a canvas child object; Also stack trace points to UI methods
+ Sprites - When sprite references are cleared from SlimeFull prefab the issue does not reproduce
+ Mono - .NET 3.5 reproduces the issue more often than .NET 4.X