Sorry, I was blame problematic prefabs and found problems caused our developer refactoring prefabs by code before one years ago. Under normal circumstances, we checked that it was rearranged.
But, still nested prefab mode in Unity2018.3 also can be trigger next exception log:
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ArgumentException:Objectof type'UnityEngine.Object'cannot be converted totype'UnityEngine.Animator'.
Reproducing steps below:
1. Create New Prefab and change to prefab mode
2. Add Blackboard/Animator components to new prefab root
3. Add Animator Blackboard Variable and link animator
4. Remove Animator