These are the most common errors you can get. It means the engine is pulling from a variable that is Null (hasn’t been set to anything yet). Sometimes this can happen even when your logic is more or less good but there is a brief moment in time where the variable hasn’t been set yet.
Use the “Is Valid” node to stop the flow of logic if a variable hasn’t been set. This can solve a lot of these types of issues. This is the Is Valid Node" unreal engine is valid - Bing
