Well, a reference to the actor is correct as for attaching / detaching.
But lots of things can happen to the object or variable in between.
This is why I asked whether you’re sure. Being sure and expecting something to be are two very different things 
You should use the BP debugging tools to verify and to be sure it’s working as intended.
Because evidently something doesn’t work as you would expect so the task now is to figure out where exactly you loose the reference. Is the object you attached destroyed somewhere? Is the reference removed or overwritten? Is something stored in a different BP instance than where you wanted it?
Lots of things can happen quite easily even to very experienced programmers!
All I can tell you from those pictures at this point is that when that error happens the variable is empty.
And that your code structure or at least the way you access things feel a bit unorganized.
Directly accessing variables and especially manipulating in other classes is not quite ideal as it makes debugging a lot more difficult, requiring you to jump across lots of classes.
Example: Right now you attach the hatchet to the player character from within a class that’s not your player character. This prevents you from storing the variable together with the actor you attached it to and leaves you without a location within the player character, where you could debug this.
I would create two functions within the player character which attach and detach the hatchet. Attaching must provide a hatchet reference. Detaching doesn’t need to. And within the player character you can then store the hatchet itself and check when and where exactly the variable is set, when it’s attached and what exactly happens to this section of code. In exactly one instance of one class.