How to reproduce:
1. Open the attached "uiToolkitEditor" project
2. Open the "Test" UI Document
3. Open the "Test" Scene
4. Observe the Console

Expected results: The warning or error informing about UI Document being assigned to the GameObject is thrown
Actual results: "IMGUIContainer cannot be used in a runtime panel." error is thrown which might be not clear because it doesn't indicate that the UI Document is assigned

Reproducible with: 6000.0.63f1, 6000.2.14f1, 6000.3.0b10, 6000.4.0a5, 6000.5.0a2

Reproducible on: macOS Tahoe 26 (M1)
Not reproducible on: No other environment tested

Note: Errors are also spawned when removing the IMGUI Containers from the UI Builders Hierarchy and each element spawns its own error