message: prevent inserting parts at index -1 for empty documents
diff: @@ -244,6 +244,10 @@ export default class EditorModel {
  addLen += part.text.length - offset;
  index += 1;
  }
  + } else if (index < 0) {
  + // if position was not found (index: -1, as happens for empty editor)
  + // reset it to insert as first part
  + index = 0;
  }
  while (str) {
  const newPart = this._partCreator.createPartForInput(str);
  