This means that while your program runs one of the function called returns the equivalent of NULL or nothing, yet your code tried to work with it nevertheless. Instead of crashing like a C program would do, UE reports this runtime error. I don’t know which one but the slot content or inventory object references are probably at fault.
You have two ways of handling this: Either add an “IsValid” node and only proceed if the returned reference is valid, or check where the object you want to access comes from and see if you are doing something wrong there. It depends whether a null return value is expected behavior or not. If you say that every character must have an inventory no matter what, you should look at the second solution and see why this one does not return an inventory (or slot), and if your setup is so that slots or inventories may or may not exist, then the first solution is probably right