FIX: fixed logic when checking for next composite part in the TreeView

[smnwttbr]

Description

Deleting the last part of a composite action raised an exception in the editor. This PR fixes this issue. ISXB-804

Changes made

The treeview logic formerly looked for the next item to process without checking if that item exists. Changes made to address this.

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:

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

[Pauliusd01]

Not reproing the issue anymore so would be willing to approve but I see there's discussion on the implementation so will hold off. Poke me if It's fine to pass and I'll approve

[simonwittber]

@ekcoh is this PR ok? IIRC we discussed accepting this PR, and scheduling other work (to add a + button to a composite) for a future time.

[ekcoh]

@simonwittber Sorry for the heavily delayed feedback. Let's land it as a fix for the bug.