Implementation of the Treap Add function

Question

Hello so I am working on my treap method which requires a Stack to track the list of nodes that it goes through, and maintains the Heap Property by doing a list of rotations at the end. However, when I run this code it gives me nullPointerException() for my rotate methods. Not sure what I am doing wrong. Here is the link to both my node and Treap class. My Node and Treap Class Code