Why do I get this error: can not set property x of undefined, when trying to load an OBJ file?

I am trying to make a gun for an FPS game with Babylon.js. I can make a shape that follows you around when you move, like you are holding a gun. Here is the code for that:

    let gun = BABYLON.MeshBuilder.CreateCylinder("rl", {diameterTop: 0.8, diameterBottom: 
    0.9, height: 3, tessellation: 64}, scene);
    gun.renderingGroupId = 1;
    gun.material = new BABYLON.StandardMaterial("rlMat", scene);
    gun.material.diffuseColor = new BABYLON.Color3(0, 0, 0);
    gun.rotation.x = Math.PI/2;
    gun.parent = camera;
    gun.position = new BABYLON.Vector3(1, -2, 5);

But that is kind of plain. So I am trying to add a OBJ gun model to make it look better.

Here is what I have so far:

let gun = BABYLON.SceneLoader.ImportMesh("gun", 
"https://dl.dropbox.com/s/4x3e5136opsm9el/", "m1911-handgun.obj", scene,);
  gun.renderingGroupId = 1;
 gun.material = new BABYLON.StandardMaterial("rlMat", scene);

gun.rotation.x = Math.PI/2;
gun.parent = camera;
gun.position = new BABYLON.Vector3(1, -2, 5);

If you try to open the URL in the browser, it won’t work because I changed it according to the docs here: Using External Assets In the Playground | Babylon.js Documentation .
But I tried the link on a different playground, so I know that it really does work.
But the gun is not there. It gives me this errorr: cannot set property of x to undefined. Please help.
Thanks for any help in advance!