I know BOX2D uses meters, and AS3 uses pixels.

I'm trying to create objects which are limited to the stageWidth.

If I do this variable:

for (var i:int = 0; i<(stage.stageWidth); i++){...} 

The animation will freeze, and this output appears:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Box2D.Collision::b2BroadPhase/CreateProxy()
at Box2D.Collision.Shapes::b2Shape/CreateProxy()
at Box2D.Dynamics::b2Body/CreateShape()
at com.actionsnippet.qbox.objects::CircleObject/build()
at com.actionsnippet.qbox::QuickObject/init()
at com.actionsnippet.qbox::QuickObject()
at com.actionsnippet.qbox.objects::CircleObject()
at com.actionsnippet.qbox::QuickBox2D/create()
at com.actionsnippet.qbox::QuickBox2D/addCircle()
at BOX2D_Test_Tutorial_fla::MainTimeline/frame1()

Does anyone know how to fix this?

Full Code:

    [SWF(width = 350, height = 600, frameRate = 60)]  
import com.actionsnippet.qbox.*;  
var sim:QuickBox2D = new QuickBox2D(this);  
sim.createStageWalls();  
// make a heavy circle  
sim.addCircle({x:3, y:3, radius:0.4, density:1});  
// create a few platforms  

// make pins 
for (var i:int = 0; i<(stage.stageWidth); i++){  
//End
     sim.addCircle({x:1 + i * 1.5, y:18, radius:0.1, density:0});
     sim.addCircle({x:2 + i * 1.5, y:17, radius:0.1, density:0});
     sim.addCircle({x:1 + i * 1.5, y:16, radius:0.1, density:0});
     sim.addCircle({x:2 + i * 1.5, y:15, radius:0.1, density:0});

//Mid end

      sim.addCircle({x:0 + i * 2, y:14, radius:0.1, density:0});
      sim.addCircle({x:0 + i * 2, y:13, radius:0.1, density:0});
      sim.addCircle({x:0 + i * 2, y:12, radius:0.1, density:0});
      sim.addCircle({x:0 + i * 2, y:11, radius:0.1, density:0});
      sim.addCircle({x:0 + i * 2, y:10, radius:0.1, density:0});

}  
sim.start();  
sim.mouseDrag();