Getting "Null Pointer Exception" Error, can't figure out why.

Hey folks,

I'm still very much a newbie at Processing––taking my first beginner class, and I'm doing this guitar tuner project using minim sounds.

I get this particular Null Pointer error if I rapidly play sounds in my sketch, but only if I click out of the sketch. The error doesn't stop my program from working, but the moment I x out of it, the error pops up in the main Processing window.

Here's the error:


And below is my main program code. I apologize for the length! I know an expert programmer could write this much more efficiently, so I apologize for my newbish-ness :-p.

1. //FONT(S):
   
   PFont b; //Size 48 Font.
   PFont c; //Size 20 Font.
   
   //MINIM:
   
   import ddf.minim.*; //import minim library.
   Minim minim; //Minim object.
   AudioPlayer [] BassPlayer = new AudioPlayer[4]; //AudioPlayer Array.
   
   //ARRAYS:
   
   Ellipses[] shadows = new Ellipses[2]; //Black ellipses that forms "shadow" of guitar.
   Quads[] qshadow = new Quads[1]; //Black quad that forms "shadow" of guitar neck.
   Ellipses[] body = new Ellipses[2]; //Guitar Body Parts.
   Ellipses[] sh = new Ellipses[1]; //Sound Hole.
   Quads[] gneck = new Quads[1]; //Guitar Neck.
   Quads[] gbridge = new Quads[1]; //Guitar Bridge.
   Points[] bpoints = new Points[4]; //Bridge Points.
   Lines[] bline = new Lines[1]; //Bridge Line.
   Lines[] gstrings = new Lines[6]; //Guitar Strings.
   Triangles[] tuners = new Triangles[4]; //Guitar Tuners.
   Ellipses[] soundbuttons = new Ellipses[4]; //Sound Buttons.
   
   void setup() {
     size(1000,700);
    
     //FONT(S);
    
       b = loadFont("ACaslonPro-Italic-48.vlw");
       c = loadFont("ACaslonPro-Italic-20.vlw");
   
   
   
   //MINIM SETUP:
   minim = new Minim(this); //IMPORT Minim Array Files.
     BassPlayer[3] = minim.loadFile("e_string.mp3"); //Imported Sounds.
     BassPlayer[2] = minim.loadFile("a_string.mp3");
     BassPlayer[1] = minim.loadFile("d_string.mp3");
     BassPlayer[0] = minim.loadFile("g_string.mp3");
    
   
   //ARRAYS:
    
     shadows[0] = new Ellipses(color(0),200,500,300,260); //"Shadow" Ellipse #1 f/Body.
     shadows[1] = new Ellipses(color(0),400,397,220,240); //"Shadow" Ellipse #2 f/Body.
     qshadow[0] = new Quads(color(0),450,312,485,377,810,180,780,120); //"Shadow" Quad f/Neck.
     body[0] = new Ellipses(color(#D3CE6B),200,485,300,260); //"Body" Ellipse #1.
     body[1] = new Ellipses(color(#D3CE6B),400,390,230,230); //"Body" Ellipse #2.
     sh[0] = new Ellipses(color(#644133),320,430,120,120); //"Sound Hole" Ellipse.
     gneck[0] = new Quads(color(#D3CE6B),450,312,485,370,810,170,780,120); //"Neck" Quad.
     gbridge[0] = new Quads(color(#644133),160,470,190,450,260,530,220,550); //"Bridge" Quad.
     bpoints[0] = new Points(178,478);//Bridge Points.
     bpoints[1] = new Points(188,490);
     bpoints[2] = new Points(197,503);
     bpoints[3] = new Points(206,516);
     bline[0] = new Lines(color(0),185,460,238,530);
     gstrings[0] = new Lines(color(#201ED3),765,140,760,130);
     gstrings[1] = new Lines(color(#0E9D52),780,170,785,180);
     gstrings[2] = new Lines(color(#C00000),175,480,680,180); //E string.
     gstrings[3] = new Lines(color(#201ED3),188,490,765,140); //A string.
     gstrings[4] = new Lines(color(#0E9D52),197,503,780,170); //D string.
     gstrings[5] = new Lines(color(#D01ED3),206,516,715,225); //G string.
     tuners[0] = new Triangles(680,180,630,150,700,110); //E tuner.
     tuners[1] = new Triangles(760,130,715,100,770,70); //A tuner.
     tuners[2] = new Triangles(785,180,780,240,850,200); //D tuner.
     tuners[3] = new Triangles(715,225,700,290,765,250); //G tuner.
     soundbuttons[0] = new Ellipses(color(#C00000),550,600,50,50);//Sound Buttons
     soundbuttons[1] = new Ellipses(color(#201ED3),650,600,50,50);
     soundbuttons[2] = new Ellipses(color(#0E9D52),750,600,50,50);
     soundbuttons[3] = new Ellipses(color(#D01ED3),850,600,50,50);
   
   }//END SETUP.
   
   void draw() {
     background(210);
     smooth();
    
     //FONT(S):
    
     textFont(b,48); //Size 48 Font.
     fill(0);
     text("Ben's Bass Guitar Tuner",10,50);
     text("E",530,550);
     text("A",630,550);
     text("D",730,550);
     text("G",830,550);
    
     textFont(c,20); //Size 20 Font.
     fill(0);
     text("Standard Guitar (E,A,D,G) tuning",10,70);
     text("To use, press corresponding buttons",575,500);
   
   
     //LOOPS:
    
     for(int i=0; i<shadows.length; i++) { // "Shadow" Ellipses.
       shadows[i].shadowCircles();
     }
    
     for(int i=0; i<qshadow.length; i++) { // "Shadow" Neck.
       qshadow[i].shadowNeck();
     }
    
     for(int i=0; i<body.length; i++) { //Guitar Body.
       body[i].bodyCircles();
     }
    
     for(int i=0; i<sh.length; i++) {  //Sound Hole.
       sh[i].soundHole();
     }
    
     for(int i=0; i<gneck.length; i++) { //Guitar Neck.
       gneck[i].Neck();
     }
    
     for(int i=0; i<gbridge.length; i++) { //Bridge.
       gbridge[i].Bridge();
     }
    
     for(int i=0; i<bpoints.length; i++) { //Bridge Points.
       bpoints[i].BridgePoints();
     }
    
     for(int i=0; i<bline.length; i++) { //Bridge Line.
       bline[i].BridgeLine();
     }
    
     for(int i=0; i<gstrings.length; i++) { //Strings.
       gstrings[i].GuitarStrings();
     }
    
     for(int i = 0; i<tuners.length; i++) { //Tuners.
       tuners[i].GuitarTuners();
     }
    
     for(int i=0; i<soundbuttons.length; i++) { //Sound Buttons.
       soundbuttons[i].sButtons();
     }
    
     //MOUSE PRESSES:
    
     if (mousePressed) { //E STRING SOUND
     if((dist(550,600,mouseX,mouseY))<50) {
       stroke(#C00000);
       strokeWeight(10);
       line(175,480,680,180);
       BassPlayer[3].play();
       BassPlayer[3].rewind();
     }
    }
    
    if (mousePressed) { //A STRING SOUND
     if((dist(650,600,mouseX,mouseY))<50) {
       stroke(#201ED3);
       strokeWeight(10);
       line(188,490,765,140);
       BassPlayer[2].play();
       BassPlayer[2].rewind();
     }
   }
   
   if (mousePressed) { //D STRING SOUND
     if((dist(750,600,mouseX,mouseY))<50) {
       stroke(#0E9D52);
       strokeWeight(10);
       line(197,503,780,170);
       BassPlayer[1].play();
       BassPlayer[1].rewind();
     }
   }
   
   if (mousePressed) { //G STRING SOUND
     if((dist(850,600,mouseX,mouseY))<50) {
       stroke(#D01ED3);
       strokeWeight(10);
       line(206,516,715,225);
       BassPlayer[0].play();
       BassPlayer[0].rewind();
     }
   }
    
   } //END DRAW.
   
   /* "void stop" prevents Minim from "freaking out"
   Without it, the "NullPointerException Error" would eventually be produced*/
   
   void stop() {
     super.stop();
   }
   

Any help would be fantastic––thanks!