I’ve been working on a template today which uses instanced static meshes and creates the level procedurally, it was going well - however I gave up on it as the ConstructionScript in UE4 is incredibly buggy and it’s absolutely impossible to work with variables using it. Testing procedural stuff without ConstructionScript so you can see changes on the fly is a pain and would take far too much time, sorry. 
You’ll just have to play around with it, trial and error is a good method of learning.
As for the error message, it’s most likely trying to cast to an object that is destroyed or is being destroyed.