getObjectId(object) {
    let key = null;
    for (key in this._objects) {
      if (this._objects.hasOwnProperty(key)) {
        if (object === this._objects[key]) {
          return key;
        }
      }
    }

    return null;
  }
Get an id by object instance
@param {fabric.Object} object object
@returns {number} object id if it exists or null