Inspectioin error

I have seen from youtube a video about inspecting, I made it - it works. Every time i try to pick up something other than the blueprints for inspection - error : K2Node_DynamicCast_AsMasterBlueprint from function “Execute Ubergraph_FirstPersonCharacter” from node : “SetActorsLocationandRotation” in graph “CollapseGraph_1” Accessed None trying to read property “K2Node_DynamicCast_AsMasterBlueprint”.
I have made a child blueprint for the objects I want to inspect, I think this is somehow the problem. (btw I am using static mesh)
The video for more details :
Part 1 :
Part 2 :

Thank you for your time !

Buuump! Anyone please?