FIX: Potential NullReferenceException in UnregisterCallbacks()

[Kevinsomnia]

Description

A callback interface can get destroyed and lead to a missing reference within the InputActions callbacks list. If SetCallbacks() is called afterwards, it can attempt to unregister null entries within the list, leading to a NullReferenceException.

Changes made

Added a null check at the top of UnregisterCallbacks().
Ran unit tests and updated auto-generated code in the Unity project.

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

[unity-cla-assistant]

All committers have signed the CLA.

[ekcoh]

Thanks for this suggested bug fix PR @Kevinsomnia. I am curious though if it's what we would like to solve this issue since invoking those functions with null in general would not be a common or recommended operation. It could make more sense to avoid invoking these methods will null in the first place. Not sure I understood exactly where the SetCallbacks(null) calls were originating from, do you have more information on that? It would likely be better to avoid calling SetCallbacks(null) in the first place or is there something I am not really understanding here?