Value is null even after i have allocated memory for the object

Hi,
I am a total newbie in java , and i am facing this issue. after allocating memory for my class instance, i am not able to access the variable in this instance.
here is the code for this.

Main function:

Code java:

  String ip=args[0];
            FileReader input = new FileReader(ip);
            BufferedReader bufRead = new BufferedReader(input);
            String line;
            int count = 0;
 
            line = bufRead.readLine();      // read first line
            int v = Integer.parseInt(line);
            //System.out.print("Value of number is "+ v);
            count++;
            g1=g1.createGraph(v,g1);

CreateGraph:

Code java:

Graph createGraph(int v,Graph g1){
        g1.numv=v;
        g1.g=new Vertex[v];
    //    g1.g[0].id=4;
        System.out.println("value of g[0].id is"+g1.g[0].id );
        for (int i=0;i<v;i++)
        {
            g1.g[i]=new Vertex();
            g1.g[i].id=i;
        }
        return g1;

The output i am getting is :

Exception in thread "main" java.lang.NullPointerException
at graphcolouring.Graph.createGraph(Graph.java:21)
at graphcolouring.GraphColouring.main(GraphColouring. java:29)