When using prefabs with an avatar descriptor, if you interact with the editor such as previewing a blink blendshape and leave the prefab mode, the avatar will stay in the preview and will throw an exception.

I've found the lip sync form to be glitchy with prefabs too.

MissingReferenceException: The object of type 'SkinnedMeshRenderer' has been destroyed but you are still trying to access it.

Your script should either check if it is null or you should not destroy the object.

AvatarDescriptorEditor3+<>c__DisplayClass71_0.<RecordBlendShape>b__0 () (at ./Packages/com.vrchat.avatars/Editor/VRCSDK/SDK3A/Components3/VRCAvatarDescriptorEditor3EyeLook.cs:619)