How to reproduce:
1. Open Unity
2. Load user-submitted project
3. Select 'W001_WaterWorld.unity'(alternatively 'R000_PresentationScene.unity') scene
4. Select any terrain polygon and edit it
5. Playtest the scene
6. Close Unity while also saving the project

Expected result: Unity saves changes and closes without errors or bug report window popping up.
Actual result: Unity saves changes but opens bug reporter window notifying the user that a crash has happened.

Notes:

• Last 'editor.log' file entry: GetManagerFromContext: pointer to object of manager 'MonoManager' is NULL (table index 5)
• Sometimes terrain polygons will render as black sprites in both scene and game views. Playing the scene sometimes makes them render again
• Terrain polygon edit triggers crash most reliably though it can be achieved editing any scene item.
• In order to run the project on some Unity versions, one might be required to resolve some errors.

Reproducible with - 5.6.2f1, 5.6.4p3, 2017.1.0a1, 2017.1.2p3, 2017.2.0a1, 2017.2.0p2, 2017.3.0a1, 2017.3.0b9, 2018.1.0a3, 2018.1.0a4
Couldn't test on:
5.6.1f1 (Unresolvable errors in the project)
2018.1.0a1, 2018.1.0a2(Due to Editor crash while trying to play the scene)