code: public GVRSceneObject getCollidable(int index)
      {
          synchronized (mCollidables)
          {
              if ((index < 0) || (index >= mCollidables.size()))
              {
                  return null;
              }
              return mCollidables.get(index);
          }
      }
docs: Get a collidabled based on its index (as returned
  by #addCollidable).
  @param index index of collidable to get
  @returns {@link GVRSceneObject} or null if not found