Pumkin
Currently if you try to build an avatar that has script components on it with a missing or invalid script on it (usually dynamic bones) it fails with the unity exception "You are trying to replace or create a Prefab from Instance that references missing script. This is not allowed." followed by a "File not found" error.
This confuses inexperienced avatar creators and causes this problem to be posted in the discord help channels literally every day.
The SDK should not allow avatars and worlds to start building if any exist in the scene (as it will fail anyway).
Since unity 2019 there's an easy way to check for and remove components with missing scripts.
Check count with:
https://docs.unity3d.com/2019.3/Documentation/ScriptReference/GameObjectUtility.GetMonoBehavioursWithMissingScriptCount.html
Autofix and remove with:
https://docs.unity3d.com/2019.3/Documentation/ScriptReference/GameObjectUtility.RemoveMonoBehavioursWithMissingScript.html