[ThinEngine] Check if document is available before accessing it

[antoine-gannat]

Description

Hi,

In ThinEngine the getHostDocument method is not react-native "safe", when accessed from a react-native application, we try to access document without checking that it exists first, thus causing the app to crash.

Repro

To repro the issue, all you need is to create a FollowCamera and make it follow a target.
I got this issue to repro by using this code in an Expo environment:

 function initGame(gl: ExpoWebGLRenderingContext) {
  const engine = new Engine(gl, true, {}, false);

  const scene = new Scene(engine);
  // Our built-in 'box' shape. Params: name, size, scene
  const box = MeshBuilder.CreateBox("box", { size: 2 }, scene);

  new FollowCamera("ArcRotateCamera", new Vector3(2, 0, 0), scene, box);
  const light = new HemisphericLight("light1", new Vector3(0, 1, 0), scene);

  // Default intensity is 1. Let's dim the light a small amount
  light.intensity = 0.7;

  scene.registerBeforeRender(() => {
    if (this.updatePan) {
      (scene.activeCamera as any).alpha -= this._translateX / 6000;
      (scene.activeCamera as any).beta -= this._translateY / 6000;
    }
    if (this.updatePinch) {
      (scene.activeCamera as any).radius -= this._scale * 60;
    }

    gl.endFrameEXP();
  });

  engine.runRenderLoop(() => {
    if (scene) {
      scene.render();
    }
  });
}

Cause

What happens here is that at some point, the Effect class (packages/dev/core/src/Materials/effect.ts) will try to access the host document by only checking if IsWindowObjectExist returns true:

const hostDocument = IsWindowObjectExist() ? this._engine.getHostDocument() : null;

This issue is, in this react-native context window will exist but not document, leading to this issue.

Fix

To fix this, I added a check in getHostDocument to make sure that document actually exist before returning it. If it doesn't, I simply return null.

This is my first PR in this repo, please let me know if I forgot anything.

[azure-pipelines]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[azure-pipelines]

Snapshot stored with reference name:
refs/pull/12571/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12571/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12571/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/12571/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/12571/merge
https://gui.babylonjs.com/?snapshot=refs/pull/12571/merge
https://nme.babylonjs.com/?snapshot=refs/pull/12571/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/12571/merge#BCU1XR#0