[Solved] Erorr with the nullpointer

You do not set next when you create your Nodes, so next is always null.

This

    Node(final Object element, Node prevNode) {
        this.element = element;
        prevNode = this;
    }

should be

    Node(final Object element, Node prevNode) {
        this.element = element;
        prevNode.next = this;
    }