Got NullReferenceException when access a nested prefab right after scene loaded from AssetBundle

I have a scene A with prefab B, and prefab C is nested inside prefab B, but it is disabled. I build scene A to an assetbundle. Of course, prefab B and C will be automatically pulled into that bundle. Right after the asynchronous loading of the scene from that assetbundle finished, if I access to prefab C inside prefab B (prefab B's Start or Awake event, for instance), I'll get a NullReferenceException. Not sure if this is a bug or I've done something incorrectly. Please correct me if I'm wrong.

 

I deleted library folder to reimport everything and the error has gone.