getLayerName (i) {
    return i >= 0 && i < this.getNumLayers() ? this.all_layers[i].getName() : ''
  }
Returns the name of the ith layer. If the index is out of range, an empty string is returned.
@param {Integer} i - The zero-based index of the layer you are querying.
@returns {string} The name of the ith layer (or the empty string if none found)