How to reproduce:
1. Open the user-submitted project
2. Open the scene "SampleScene" (Assets/Scenes/SampleScene.unity)
3. In the Hierarchy window, select the GameObject "Canvas" and enter Play mode
4. While Play mode is loading, start clicking the left mouse button inside the Scene view
5. When Play mode has loaded, inspect the Console window (Window > General > Console)

Expected result: No errors are seen
Actual result: GUI and NullReferenceException errors are seen

Reproducible with: 2021.2.0b13, 2021.2.15f1
Not reproducible with: 2019.4.36f1, 2020.3.31f1, 2021.2.0b12, 2022.1.0a1, 2022.1.0b11, 2022.2.0a7

Note: Might take a few tries to reproduce. Retry from step 3 if the errors didn't appear.

Errors:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.SceneView.CallOnPreSceneGUI () (at /Users/bokken/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:3659)

GUI Error: You are pushing more GUIClips than you are popping. Make sure they are balanced.
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&) (at /Users/bokken/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:189)