Oh my god… I somehow missused the interface… I gave up on it and just removed it and now it works perfectly fine!!! 
I guess somehow the interface didnt get the BuildingToBuild variable and so in Ghost Building it was set to null and so it replaced the variable of BTB AFTER Ghost got it from the townhall (first picture in last reply) and so it was set to NULL right before “Spawn actor from class” node 