Visual Studio opens up instead of UI Builder on double-clicking on uXML file in the Project window.
Workaround: Open the uXML file from Viewport > File > Open.

Steps to repro:
1. Create a new Project.
2. Window > UI Toolkit > UI Builder.
3. Viewport > File > Save > File name > TestuXML > Save.
4. Close UI Builder Window.
5. Project Window > Double Click on TestuXML file.

Actual Result:
Visual Studio opens up instead of UI Builder window.

Expected Result:
UI Builder window opens up with the correct document.

Reproducible in:
2021.1.0a9, 2021.1.0a1.

Working Fine in:
2020.2.0f1.

Environment:
Occurring only on Windows.