referencing Hands of leap Motion

For research I want to make a little game in order to see how quick probands react to moving objects. Therefor I want to place a Target in a defined room. The Target shall appear in different positions and the user shall catch it. The movements I want to track with the Leap motion.
I already made the room and the target to catch. When the target is touched by the hand I want the target to change color and then to move to another place while the hands need to be put back at the starting positions.
Since weeks I am not able to fix it. First the BoxCollision of the Target did not respond when the hands where colliding. Now I tried to check if the hands reached the target by checking the worlLocation of Target and hands but somehow the reference in my BP_Target and BP_Room seem to be null. Can anybody explain me why this is the case?
How can I reference the right and left hand in an appropriate way?

Here you see the error message.


I marked where exactly the error occurs.


I thank you very much in advance!