Bug: Deleting grabbed objects causes nullrefs

[MattGrayUL]

Contributor Tasks

• [x] Create or edit test cases here
• [x] Add a CHANGELOG entry for this change.
• [x] Ensure documentation requirements are met e.g., public API is commented.
• [x] Consider any licensing/other legal implications for this MR e.g. notices required by any new libraries.

Test Cycle

Link to the test cycle here.

Reviewer Tasks

• [ ] Code reviewed.
• [ ] Non-code assets e.g. Unity assets/scenes reviewed.
• [ ] All tests must be ran and cover all scenarios (If not, add new tests to the cycle and run them).
• [ ] Documentation has been reviewed.
• [ ] Approve with a comment of any additional tests run or any observations.

Related JIRA Issues

Closes UNITY-1474 Closes UNITY-1472