How to reproduce:
1. Create a new project with a 2D URP template
2. In the Hierarchy window right-click > Camera
3. Select "Camera" GameObject and change Render Type to "Overlay"
4. Select "Main Camera" GameObject and add "Camera" to the Stack list
5. Enable Post Processing
6. Enter the Play Mode
Expected result: No warnings
Actual result: Warning appears: "CommandBuffer: temporary render texture _InternalGradingLut not found while executing (SetGlobalTexture) UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)"
Reproducible with: 12.0.0 (2021.2.0b15), 12.1.2 (2021.2.7f1), 13.1.3 (2022.1.0b2)
Could not test with: 2019.4.34f1, 2020.3.25f1 2021.2.0a1 (No 2D URP template)
Note: Not reproducible in 3D URP template