Fishnet version : 4.3.7R.Pro
When using Yak, I'm getting a lot of Error: "{thing} is a prefab, . You must instantiate the prefab first, then use Spawn on the instantiated copy." when stopping playmode.
It seems to happen only on global objects, and is probably a race condition with unity unloading scenes when stopping playmode, causing networkObject.gameObject.scene to be null even though the object is not a prefab.