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

[MnogoZnaet]

I found this issue and fix it. My case:
I have multiple scenes with portals. LayerManager defined as singleton as well as definedDimensions list is static. So the problem is when I unload scene, all dimensions that was in the scene deleted, and now definedDimensions variable contains "null". What I offer is to use LayerManager as MonoBehaviour Singleton so it is related to the scene and remove static from definedDimensions so it also related to LayerManager instance.