showContentByClass(layerClass, withoutOverlay) {
    if (layerClass != null && !(this._contentContainer.getCurrentLayer() instanceof layerClass)) {
      return this.showContent(new layerClass(), withoutOverlay);
    }
    if (withoutOverlay && this._overlay.getCurrentLayer() != null) {
      this._contentPromise = Promise.all([this._contentPromise, this.destroyOverlay()]);
    }
    return this._contentPromise;
  }
Shows content layer if it not already showing content of layerClass and returns a promise that resolves when the content has been fully swapped.
@param layerClass
@param [withoutOverlay=false] whether to clear overlay
@returns {Promise}