Reproduction steps:
1. Open the attached “CrashRepro” project
2. In the Project window, click “Assets/ThirdParty/EMD Assets/CharacterPack Lowpoly/Models/character1.fbx”
3. In the Inspector window, click Rig
4. Change the Animation Type to “Generic” and click Apply
5. Change Avatar Definition to “Copy From Other Avatar”
6. Click Source and select “AjAvatar”
7. In the prompt, click Yes

Expected result: Animation Type is set to “Humanoid”
Actual result: Unity Editor crashes

Reproducible with: 2020.3.44f1, 2021.1.4f1, 2021.1.28f1, 2021.2.0a5, 2021.2.19f1, 2021.3.16f1, 2022.1.24f1, 2022.2.2f1, 2023.1.0a25
Not reproducible with: 2021.1.0a1, 2021.1.3f1, 2021.2.0a4
Couldn’t test with: 2021.2.0a1 - 2021.2.0a3 (Couldn’t test because none of the Avatars are displayed in “Select Avatar” window)

Reproducible on: Ubuntu 20.04
Not reproducible on: Windows 11 22H2, MacOS 13.0 Ventura

First lines of the stack trace:

{noformat}#0 0x0056514490831c in burst_signal_handler(int, siginfo_t*, void*)
#1 0x007f5f39a92420 in funlockfile
#2 0x00565145ebe9e0 in GUIView::SendLayoutEvent(GUIState&)
#3 0x00565145ebee20 in GUIView::OnInputEvent(InputEvent&)
#4 0x005651458926bb in GUIView::ProcessInputEvent(InputEvent&)
#5 0x00565145ec2ea1 in OnGUIViewButtonPress(GUIView*, void*)sss{noformat}