Sticky note cannot be assigned to a group after its assigned group has been deleted

Steps to reproduce:

1. Import the attached VFX
2. Delete 'Gruop' group 
3. Assign 'Sticky' sticky note to the 'Another group' group

Actual results: Sticky note cannot be assigned to 'Another group' group

Expected results: Sticky note can be assigned to 'Another group' group

Reproducible with versions: 6000.3.0a1, 6000.3.0b5, 6000.4.0a3

Not reproducible with versions: n/a

Can’t test with versions: n/a

Tested on (OS): macOS

Notes: Introduced by the sticky notes feature, which added a new option to assign sticky notes to a group, hence it's not a regression.

•