Edit (thomas.lopez): Problem appears here in Polybrush but it is coming from IMGUI Gradient in the Editor window and not Polybrush related.

Steps to repro:
1. Open the attached project "3DGameKit-URP-2021.2-RTGI" and "BakedIndirect" scene;
2. Go to Preferences window (Edit>Preferences...) and click on the Polybrush tab.

Expected result:
The Polybrush tab opens without errors in the Console.

Actual result:
The error: Assertion failed on expression: 'didInsert'
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)
is shown in the Console. See the attached video.

Regression introduced in Version 2021.1.

Notes:
- Reproducible in Version 2022.1.0a2, 2021.2.0b4, 2021.1.15f1;
- Not reproducible in Version 2020.3.14f1;
- Reproducible on Windows.