add(ctrlToAdd) {
    if (this.list.find(ctrl => ctrlToAdd === ctrl)) {
      throw new Error('controller instance is already added');
    }
    this.list.push(ctrlToAdd);
    return ctrlToAdd;
  }
Registers an overlay controller.
@param {OverlayController} ctrlToAdd controller of the newly added overlay
@returns {OverlayController} same controller after adding to the manager