Removing a type in Theme Editor's Manage Items dialog removes all types instead of just the one clicked

[TX256]

Tested versions

Reproducible in (at least): v4.4.dev7.official [46c8f8c]

System information

Windows 11

Issue description

Simply put, when I click the trash icon next to a type item in the Manage Items dialog (Theme Editor), it removes ALL items, instead of just the one I clicked.

Steps to reproduce

1. Create a new Theme resource
2. Add at least 2 types with the + icon
3. Click "Manage Items..."
4. Click "Remove Item" trash icon next to any item
5. Notice how all items are removed

Minimal reproduction project (MRP)

N/A