Need Help fixing NullPointerException error in my code

Question

im a biginner in java ,im trying to create a rubber band dynamic graph but when i ran my program it came back with these errors : Exception in thread "main" java.lang.NullPointerException

• at Node.add_neighbour(Node.java:58)
• at Graph.add(Graph.java:32)

public class Node {

    private static final double FRICTION = -0.8;
    Vector position;
    Vector velocity;
    Set<Node> Neighbours;
    boolean Mobile, Rigid;

    public Node(Vector vector) {
        Vector vector1 = new Vector(0, 0);
        this.Rigid = false;
        this.Mobile = true;
        Set<Node> myNodes = new HashSet<>();
    }

    public void add_neighbour(Node node) {
        Neighbours.add(node); //**the 1st error is indicated in this line**
    }
}

2nd error:

import java.util.HashSet;
import java.util.Set;

public class Graph {

    Set<Node> nodes = new HashSet<>();

    public Set<Node> getNodes() {
        return nodes;
    }

    public void add(Node node) {
        nodes.add(node);
    }

    public void add(Node node1, Node node2) {
        node1.add_neighbour(node2);//2nd***
        node2.add_neighbour(node1);//2nd****
    }
}