How to reproduce:
1. Open the attached Unity project or create a new one
2. Open Sequences window
3. Create child sequences until one of them is marked red
4. Create one more sequence
5. Observe that the last sequence is not created
6. Try to delete the parent sequence
7. Change focus to a Console or Hierarchy window

Expected result: Sequences are deleted with no errors
Actual result: NullReferenceException appears

Reproduced with: 1.0.2 (2022.1.0a8, 2021.2.0b12)

Note: After step 3, when one of the sequences becomes red, you can see that some sequences disappear from the Sequences window.