Game Studio crashes on close with a NRE in ScriptSystem

[dfkeenan]

Release Type: Official Release and GitHub

Version: Tested in 3.1.0.1-beta02-550 and master branch

Platform(s): Windows (Game Studio)

Describe the bug
There are certain situations in which a NullReferenceException is throw in ScriptSystem.AddTask when closing Game Studio. It appears that AddTask is being called in GameStudioPreviewService.SetAssetToPreview after the ScriptSystem has been "destroyed" so the property ScriptSystem.Scheduler is null.

To Reproduce
Steps to reproduce the behavior:

1. Setup Game Studio so you have one Scene open and the Output window open (see below) and close it.
2. Open project in Game Studio.
3. Select entity in the hierarchy.
4. Close Game Studio

Expected behavior
Game Studio should not crash on closing.

Screenshots

Log and callstacks
If any, please attach here any log or callstack (preferably in a .txt file using GitHub drag and drop)

Additional context
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