From 'Text' windows, when 'Font Asset Creator' or 'Sprite Importer' window is opened then on enabling 'UI Toolkit Live Reload' option that particular window turns blank with Script not found in project message.
Refer attached video.

Steps to repro:
1. Create a new project.
2. Window > Text
3. Text > 'Font Asset Creator' or 'Sprite Importer' > Enable 'UI Toolkit Live Reload' option

Actual Result:
Windows Failed to load on enabling 'UI Toolkit Live Reload' option in 'Text' windows.

Expected Result:
Windows load without any fail after enabling 'UI Toolkit Live Reload' option in 'Text' windows.

Occurring on:
2021.2.0a7, 2021.2.0a4

Workaround: Reopening of 'Font Asset Creator' or 'Sprite Importer' windows will fix the issue.

Note:
1] 'Text' windows separately introduced in 2021.2.0a4
2] On enabling 'UI Toolkit Live Reload' option in 'Font Asset Creator' or 'Sprite Importer' from the 'TextMeshPro' window it works fine, without any 'Windows Failed to load' issue.

Environment:
Windows 10 & macOS 10.15