FIX: "ArgumentNullException: Value cannot be null." when deleting a newly created input action (ISXB-1405)

[jfreire-unity]

Description

Fixes https://jira.unity3d.com/browse/ISXB-1405.

The problem ocurred when creating a new action and deleting it before finishing "renaming it". The delete triggers a focus change which calls the event to "rename" the action.
Since at this stage, the action is in "renaming mode" (m_IsEditing == true) and the action was deleted, the search for that action serialized property would return "null". That case was not being considered before.

Testing status & QA

Local testing on macOS trying to replicate the problem.
Creating new actions with a name and removing them also works.

Overall Product Risks

Please rate the potential complexity and halo effect from low to high for the reviewers. Note down potential risks to specific Editor branches if any.

• Complexity: 0
• Halo Effect: 0

Comments to reviewers

I'd only ask to do some more exploratory work around renaming and creating actions in both Project Settings > Input System and a standalone Asset Editor view.

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.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• 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.

After merge:

• Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.

[ritamerkl]

Looks good! Thanks