neo4j sorting nodes in java according to degree

I need to sort nodes based on degree that are created by my program on the fly.

I tried to use Collections.sort() but it throws a null pointer exception. The nodeList parameter is list of nodes created by my program. I tried few different configurations, but it did not worked.

private  void sortNodes(List<Node> nodeList){

    Collections.sort(nodeList, new GraphModel());
}

public int compare(Node n1, Node n2) {

    int n1Degree,n2Degree;

    try ( Transaction tx = graphDb.beginTx() ) {
        n1Degree = n1.getDegree();
        n2Degree = n2.getDegree();
        tx.success();

    }
    if(n1Degree<n2Degree){
        return -1;
    }
    else if(n1Degree==n2Degree){
        return 0;
    }
    else{
        return 1;
    }

}