How to reproduce:
1. Create a new project
2. Create 2 UI Document files (Create -> UI Toolkit -> UI Document)
3. Open one of the UI Documents
4. Add 2 of the other UI Document in the UI Builder Hierarchy (Library -> Project -> Assets -> Drag into Hierarchy)
5. Unpack both UI Documents (Right Click -> Unpack Instance)
6. Observe the Console Window and the UI Builder Window
Expected result: UI Documents get unpacked
Actual result: Only the first UI Document gets unpacked and the second one throws a NullReferenceException error
Reproducible with: 6000.0.60f1, 6000.2.0a7, 6000.2.0b2 (2bdac9ac0d74), 6000.2.10f1
Not reproducible with: 6000.2.0a6, 6000.3.0b7, 6000.4.0a4
Fixed in: 6000.3.0a1
Reproducible on: Windows 11 Pro (24H2)
Not reproducible on: No other environments tested