Reproduction steps:
1. Open the attached “IN-114385.zip” project
2. From the Project window, double-click on the “Test.uxml” to open in UI Builder
3. In the Hierarchy, select the “#WidgetInstantiator”
4. In the Inspector, under Attributes, expand the “Widget Test” list
5. Click the + icon to add new item to the list
6. Observe the Console

Expected result: No errors and list items are saved
Actual result: “NullReferenceException: Object reference not set to an instance of an object” is thrown and list items are not saved

Reproducible with: 6000.1.0a9, 6000.2.4f1, 6000.3.0b1
Not reproducible with: 6000.0.57f1, 6000.1.0a8

Reproducible on: Windows 11
Not reproducible on: no other environments tested

Note:
- Lists of primitive types (e.g., List<int>) work correctly