Java.lang.NullPointerException how can I fix it?

I'm new to processing This is my code below pls tell me what's wrong with my code

            import shiffman.box2d.*;
            import org.jbox2d.collision.shapes.*;
            import org.jbox2d.common.*;
            import org.jbox2d.dynamics.*;

            short count, dim = 150;
            int num = 0;

            PFont counter;

            final static short POS = 25, FPS = 50, BOLD = 0;
            final static color  BG = #C04000,
            FG = #82CFD0, BORDER = 0;



            PImage photo;


            // A reference to our box2d world
            Box2DProcessing box2d;

            // A list we'll use to track fixed objects
            ArrayList<Boundary> boundaries;
            // A list for all of our rectangles
            ArrayList<CustomShape> polygons;

            ArrayList<Ball> balls;

            int missed =0;

            void setup() {
              size(1000,600);
              smooth();
              counter = createFont("Arial", 72, true);

              photo = loadImage("2-01.png");

              // Initialize box2d physics and create the world
              box2d = new Box2DProcessing(this);
              box2d.createWorld();
              // We are setting a custom gravity
              box2d.setGravity(0, -10);

              // Create ArrayLists
              balls = new ArrayList<Ball>();
              boundaries = new ArrayList<Boundary>();
              polygons = new ArrayList<CustomShape>();

              // Add a bunch of fixed boundaries
              // b1
              Boundary b1 = new Boundary(200f, 200f, 150f, 10f, radians(45));
              boundaries.add(b1);

              // b2
              Boundary b2 = new Boundary(500f, 200f, 150f, 10f, radians(-45));
              boundaries.add(b2);

              Boundary b3 = new Boundary(100f, 200f, 150f, 10f, radians(0));
              boundaries.add(b3);


              //right side
              Boundary rightSide = new Boundary(width-5,height/2,10,height,0);
              rightSide.setRotation(-PI/8);
              boundaries.add(rightSide);
              // left sides
              Boundary leftSide = new Boundary(5,height/2,10,height,0);
              leftSide.setRotation(PI/8);
              boundaries.add(leftSide);
            }

             {
              size(208, 184);
              photo = loadImage("2-01.png");
            }


            void draw() {
              background(255);
              // We must always step through time!
              box2d.step();

              image(photo,mouseX,0);
              // Display all the boundaries
              for (Boundary wall: boundaries) {
                //wall.setRotation(PI/6);
                wall.display();
              }

              // Display all the people
              {
                //for (Ball b: balls)
               for (int i=0; i<balls.size(); i++)
               {
                 //important
                Ball b = balls.get(i);
                b.display();
                Vec2 pos   = box2d.getBodyPixelCoord(b.body);
                if(b.done())
                {
                  missed++;
                  balls.remove(i);
                  println("ball missed:"+missed);
                  createBall();
                }
               }
              }



              {
                //for (Ball b: balls)
               for (int i=0; i<balls.size(); i++)
               {
                 //important
                Ball b = balls.get(i);
                b.display();

                 image(photo,mouseX,0);

                if(b.done())
                {
                  missed++;
                  balls.remove(i);
                  println("ball missed:"+missed);
                  createBall();
                }
               }
              }