FIX: Avoid potential for NullReferenceException in FireStateChangeNotifications (ISXB-724).

[graham-huws]

Description

A tentative fix, given no known repro, for ISXB-724.

Changes made

Added early returns in cases we'd otherwise crash.

Checklist

Before review:

• [x] 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:

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

[graham-huws]

@stefanunity I realise there's nothing really to test here given that there's no repro steps for the original issue, but just wanted you aware of this regardless in case you had anything to add 😃

[graham-huws]

N.B. tvos failure is also happening on develop branch.