Reproduction steps:
1. Open project “UITKTest.zip”
2. Open “UXML2.uxml” with UI Builder
3. Switch to Project view in the control's Library
4. Unfold everything in the “UI Documents (UXML)” and “Custom Controls (C#)”
5. Drag “CustomControl” over to “Assets/Scripts/UI/Resources/UXML1.uxml”

Expected result: “CustomControl” is dragged over “UXML1.uxml”
Actual result: the Editor crashes

Reproducible with: 2023.2.0a17
Couldn’t test with: 2021.3.26f1, 2022.3.0f1, 2023.1.0b19 (compiler errors and custom controls have disappeared)

First few lines of the Stack Trace:
0x00007FF9B9C334B8 (Unity) pqInit
0x00007FF9B9C34447 (Unity) tessComputeInterior
0x00007FF9B9C36722 (Unity) tessTesselate
0x00007FF9BAA76128 (Unity) UIToolkit::UIPainter2D::TessellateFillWithArcMappings
0x00007FF9BAA6D7F2 (Unity) UIToolkit::UIPainter2D::FillInternal