How can I control my particles in one window from another window via the mouse?

I am not sure if I am missing something specific to the toxi library, but I keep getting a ConcurrentModificationException when I try to control my particles in my secondary window via the mouse in my primary window. The crash seems to be random, sometimes it crashes on one click, sometimes I can drag the particles around for a second or two either way I can;t seem to figure it out. Can anyone help, please?

Particles p;
AttractionBehavior mouseAttractorP;
Vec2D mousePos;
FinalPanel tp;

void setup() {
  size(1024, 768, P3D);
  frameRate(60);
  tp = new FinalPanel(400,400);
  surface.setTitle("Touch Panel");

}

void draw() {
  background(0, 0, 0);
}



void mousePressed() {
  mousePos = new Vec2D(mouseX, mouseY);

  if (mouseButton == LEFT) {
    // create a new attraction force field around the mouse position
    mouseAttractorP = new AttractionBehavior(mousePos, 250, 7);
    p.physics.addBehavior(mouseAttractorP);
  }

  if (mouseButton == RIGHT) {
    // create a new repulsion force field around the mouse position
    mouseAttractorP = new AttractionBehavior(mousePos, -33, -1);
    p.physics.addBehavior(mouseAttractorP);
  }
}

void mouseDragged() {
  // update mouse attraction focal point
  mousePos.set(mouseX, mouseY);
}

void mouseReleased() {
  // remove the mouse attraction when button has been released
  p.physics.removeBehavior(mouseAttractorP);
}


import toxi.geom.*;
import toxi.physics2d.*;
import toxi.physics2d.behaviors.*;
import toxi.math.*;
import megamu.mesh.*;



class Particles {
  final PApplet g;
  int NUM_PARTICLES;
  VerletPhysics2D physics;
  AttractionBehavior mouseAttractor;
  float [][] pos;
  float eSize;
  float xOff;

  Delaunay delaunay;
  int [] polygons;


  Particles(PApplet _p) {
    g = _p;
    xOff = 0;


    NUM_PARTICLES = 666;
    eSize = 1;
    physics = new VerletPhysics2D();
    physics.setDrag(0.7);
    physics.setWorldBounds(new Rect(0, 0, width, height));
    physics.addBehavior(new GravityBehavior(new Vec2D(-0.00007, -0.0003f)));
    for (int i = 0; i<NUM_PARTICLES; i++) {
      addParticle();
    }
  }


  void addParticle() {
    VerletParticle2D p = new VerletParticle2D(Vec2D.randomVector().scale(133).addSelf(width / 2, height/2));
    physics.addParticle(p);
    // add a negative attraction force field around the new particle
    physics.addBehavior(new AttractionBehavior(p, 13, 0.3f, 3f));
  }

  void drawParticles() {
    // store particles delaunayitions to do delaunay triangulation
    pos = new float[NUM_PARTICLES][2];

    for ( int i=0; i<NUM_PARTICLES; i++) {
      // particle system using verlet integration
      VerletParticle2D p = physics.particles.get(i);
      g.pushStyle();
      g.fill(255, 33);
      g.ellipse(p.x, p.y, g.noise(xOff) * 13, g.noise(xOff) * 13);
      g.popStyle();

      pos[i][0] = physics.particles.get(i).x;
      pos[i][1] = physics.particles.get(i).y;
    }
  }


  // delaunay triangulation logic taken from here :
  // http://www.openprocessing.org/sketch/43503
  void drawLines() {
    // delaunay triangulation
    delaunay = new Delaunay(pos);
    // getEdges returns a 2 dimensional array for the lines
    float[][] edges = delaunay.getEdges();
    for (int i=0; i<edges.length; i++)
    {
      // use the edges values to draw the lines
      float startX = edges[i][0];
      float startY = edges[i][1];
      float endX = edges[i][2];
      float endY = edges[i][3];
      float distance = dist(startX, startY, endX, endY);
      // remap the distance to opacity values
      float trans = 255-map(distance, 0, 33, 33, 0);
      // stroke weight based on distance
      // fast invert square root helps for performance
      float sw = 3f/sqrt(distance*2);
      g.ellipseMode(CENTER);

      g.pushStyle();
      g.strokeWeight(sw);
      g.stroke(g.noise(1)*233);
      g.fill(g.random(255), g.random(255), g.random(255));
      g.noFill();
      g.ellipse(startX, startY, eSize, eSize);
      g.popStyle();
      //pushStyle();
      //stroke(200, 3);
      //noFill();
      //bezier(startX, startY, endX, mouseY, width/2, mouseY, width/2, height/2);
      //popStyle();
    }
  }
}



class FinalPanel extends PApplet {
  int w, h;

  public FinalPanel(int _w, int _h) {
    super();
    w = _w;
    h = _h;
    PApplet.runSketch(new String[]{this.getClass().getName()}, this);
  }

  public void settings() {
    size(w, h, P3D);
  }

  public void setup() {
    blendMode(ADD);
    surface.setTitle("Final Panel");
    surface.setLocation(0, 0);
    p = new Particles(this);
  }

  void draw() {
    background(0);
    p.physics.update();
    p.drawParticles();
    p.drawLines();
  }
}