message: Fix index for _insertMention
diff: @@ -326,7 +326,8 @@ export default class SendMessageComposer extends React.Component {
  member.rawDisplayName : userId;
  const caret = this._editorRef.getCaret();
  const position = model.positionForOffset(caret.offset, caret.atNodeEnd);
  - const insertIndex = position.index + 1;
  + // index is -1 if there are no parts but we only care for if this would be the part in position 0
  + const insertIndex = position.index > 0 ? position.index : 0;
  const parts = partCreator.createMentionParts(insertIndex, displayName, userId);
  model.transform(() => {
  const addedLen = model.insert(parts, position);
  