fix(scripting/mono-v2): validate app domain before removing ref

[DaniGP17]

Goal of this PR

Fix a crash that occurred when an export declared in lua code was executed from C# v2 from the same resource multiple times, the issue with more details is here: thorium-cfx/mono_v2_get_started#32

How is this PR achieving the goal

It seems that there are times that m_appDomain is destroyed when the script is stopped, but RemoveRef is still called a little after the C# script is destroyed, therefore RemoveRef is called when m_appDomain is null and results in the crash.
I don't know if there is a better solution for this.

This PR applies to the following area(s)

ScRT: C#

Successfully tested on

Game builds: .

Platforms: Windows

Checklist

• Code compiles and has been tested successfully.
• Code explains itself well and/or is documented.
• My commit message explains what the changes do and what they are for.
• No extra compilation warnings are added by these changes.

Fixes issues

fixes #32