NullPointer Exception Issue

I have a code that I need running for several hours. After several minutes it stops responding altogether (and I have to force quit the application) and gives me the error "NullPointer Exception" and highlights line 106. I have not been able to solve this, it may be because it runs out of pixels to grab? I am not sure. Another issue I am having is that it is using both my USB camera and my built-in mac camera. How do I get it to only use the USB camera? final int kTotalGlitches=10; int currGlitch=0; PImage img; ArrayList glitches;

final int kGlitchState=0;
final int kCamState=1;

int state=kCamState;


// ---------------------------------

final int kGlitchTimeLapse=7000;  // 2 secs
final int kCamTimeLapse=5000;    //8 secs
int triggerTime;

// ---------------------------------


import processing.video.*;
Capture cam;
Capture video;



void setup() {
  size(1024, 768);
  imageMode(CENTER);

  glitches = new ArrayList <GlitchClass>();
  for (int i=0; i<kTotalGlitches; i++)
    glitches.add(new GlitchClass());

  cam = new Capture(this);
  cam.start();
  triggerTime=millis()+kCamTimeLapse;
  img = cam;
  video = new Capture(this, width, height);
  video.start();
}

void draw() {

  if (millis()>=triggerTime) {

    if (state==kCamState) {
      state=kGlitchState;
      triggerTime+=kGlitchTimeLapse;
      currGlitch=int(random(kTotalGlitches));  //Select new glitch
    } else {
      state=kCamState;
      triggerTime+=kCamTimeLapse;
    }
  }

  if (state==kCamState)
    image(cam, width/2, height/2);
  else {
    background(0);
    glitches.get(currGlitch).display();
  }
}


void captureEvent(Capture camera) {
  camera.read();

  img = createImage(width, height, RGB);
  video.loadPixels();
  arrayCopy(video.pixels, img.pixels);
}


//A glitch is just two ellipses overlapping on top of each other with different
//colors and one being rotated by 90 degrees.
class GlitchClass {

  void display() {

    loadPixels();

    int randPos = 0;
    if (frameCount  % 100 == 0)
      randPos = (int)random(0, video.height -4);

    int randPosY = 0;

    // if (frameCount  % 100 == 0)
    randPosY = (int)random(20, 50);

    // Begin a loop for displaying pixel rows of 4 pixels height
    for (int y = 0; y < video.height -57; y++) {
      if (img != null) {
        img.loadPixels();

        // Put 4 rows of pixels on the screen
        if (frameCount % 50 == 0)
          randPosY = (int)random(20, 50);
        for (int x = 0; x < video.width; x++) {
          if (frameCount % 100 == 0)
            randPosY = (int)random(20, 50);
          if (y < video.height -4) {
            pixels[x + (y + 0 + randPosY)* width] = img.pixels[  (y + 0 )* video.width + randPos + x];
            pixels[x + (y + 1 + randPosY) * width] = img.pixels[ (y + 1) * video.width + randPos + 1 + x];
            pixels[x + (y + 2 + randPosY) * width] = img.pixels[ (y + 2) * video.width + randPos + 2 + x];
            pixels[x + (y + 3 + randPosY) * width] = img.pixels[ (y + 3) * video.width + randPos + 3 + x];

            pixels[x + (y + 4 + randPosY)* width] = img.pixels[  (y + 4 )* video.width + randPos + 4 + x];
            pixels[x + (y + 5 + randPosY) * width] = img.pixels[ (y + 5) * video.width + randPos + 5 + x];
            pixels[x + (y + 6 + randPosY) * width] = img.pixels[ (y + 6) * video.width + randPos + 6 + x];
            pixels[x + (y + 7 + randPosY) * width] = img.pixels[ (y + 7) * video.width + randPos + 7 + x];
          }
        }
      } else {
        break;
      }
    }

    updatePixels();
  }
}

Thanks!