setLastRange(rng) {
    if (rng) {
      this.lastRange = rng;
    } else {
      this.lastRange = range.create(this.editable);

      if ($(this.lastRange.sc).closest('.note-editable').length === 0) {
        this.lastRange = range.createFromBodyElement(this.editable);
      }
    }
  }
set the last range

if given rng is exist, set rng as the last range
or create a new range at the end of the document

@param {WrappedRange} rng