Yep, i guess you try to use a reference variable that is still NULL. Make sure the variable you use can’t be NULL. Either by using “IsValid” node with it or by making sure that you filled it before it is used.
A variable that takes in an actor(object) is something different than an int variable. It cann be empty and if you try to use some from this variable, although it is still NULL, you will get this error.
Look at what Variable causes the error and use the IsValid node. If the error disappears, then try to look if the variable is already set to something before using it.
Sometimes you can call function before the actors is initialized. So waiting a few secs will make sure that everything works (loading screens etc).