I've done the coding, however I'm getting a NullPointerException error. All I want is for whenever the user clicks the mouse, it plays a sound on a loop, and when the mouse is released the sound stops. What am I doing wrong?

Much appreciated.

1. first circle_;
   second_[] smallcircle = new second_[4000];
   
   import ddf.minim.* ;
   Minim minim;
   
   AudioPlayer au_player1 ;
   
   // Button Features
   boolean knobHighlight1 = false;
   boolean knobHighlight2 = false;
   float knob1, knobOne, knob2, knobTwo;
   int xKnob1;
   int yKnob1;
   int yKnob2;
   int knobMass;
   PImage bg;
   
   void setup() {
   size(640,480);
   
   minim = new Minim(this);
   au_player1 = minim.loadFile("bubbles.wav");
   
   textSize(30);
   text("Fill My World With Colour",50,50);
   textAlign(CENTER,CENTER);
   
   circle_ =new first ();
   
   for (int s = 0; s <100; s++) {
     circle_ = new first();
     smallcircle[s]=new second_();
   }
   
   knob1 = color(350);
   knobOne = color(230);
   knob2 = color(350);
   knobTwo = color(230);
   xKnob1=470;
   yKnob1=10;
   yKnob2=40;
   knobMass=10;
   
   }
   
   void draw() {
   update(mouseX, mouseY);
   
     // Knobs Are Lit With Colour When Highlighted By Mouse
     if (knobHighlight1) {
       fill(knob1);
             
       knob1pressed(); // Function For Chosen Knob
     }
     else {
       fill(knobTwo);
     }
     ellipse (xKnob1, yKnob1, knobMass, knobMass);
   
     if (knobHighlight2) {
       fill(knob2);       
   
       knob2pressed();
     }
     else {
       fill(knobTwo);
     }
     ellipse (xKnob1, yKnob2, knobMass, knobMass);
   }
   
   // Boolean To Cause Action Once Mouse Is Pressed
   void update(int x, int y) {
     if ( knobHighlight1(xKnob1, yKnob1, knobMass)) {
       knobHighlight1 = true;
       knobHighlight2 = false;
     }
     else if ( knobHighlight2(xKnob1, yKnob2, knobMass) ) {
       knobHighlight1 = false;
       knobHighlight2 = true;
     } 
   }
   
   // Position of the Knobs
   boolean knobHighlight1(int x, int y, int diameter) {
     float xRange = x - mouseX;
     float yRange = y - mouseY;
     if (sqrt(sq(xRange) + sq(yRange)) < 15 ) {
       return true;
     }
     else {
       return false;
     }
   }
   
   
   boolean knobHighlight2(int x, int y, int diameter) {
     float xRange = x - mouseX;
     float yRange = y - mouseY;
     if (sqrt(sq(xRange) + sq(yRange)) < 15 ) {
       return true;
     }
     else {
       return false;
     }
   }
   
   void knob1pressed() {
     if (mousePressed==true) {
       fill(0);
       circle_.draw_();
       noStroke();
       fill(random(0, 255));
     }
   }
   void knob2pressed() {
     if (mousePressed==true) {
       for (int s = 0; s < 50; s++) {
         fill(0);
         smallcircle[s].draw_();
         smallcircle[s].move();
       }
     }
   }
   
   void mousePressed(){
     au_player1.play();
     au_player1.loop();
   }
   
   void mouseReleased(){
     au_player1.close();
     minim.stop();
   }