Random generation questions

So I have a script going in the level blueprint, and I have four variants of my stairwell done as sublevels. The script is set up supposedly to generate a downward line of randomly chosen sublevels, but I have some problems:
-When I try to play the level, I get a blueprint runtime error saying my Create Instance node is accessing “None”
-I can’t find a way to make it loop endlessly
-I don’t really know how the random seeds work

Screenshots: