message: prevent exception if there is no text node at anchor
diff: @@ -382,7 +382,8 @@ export default class MessageComposerInput extends React.Component {
  const quote = Block.create('block-quote');
  if (this.state.isRichTextEnabled) {
  let change = editorState.change();
  - if (editorState.anchorText.text === '' && editorState.anchorBlock.nodes.size === 1) {
  + const anchorText = editorState.anchorText;
  + if ((!anchorText || anchorText.text === '') && editorState.anchorBlock.nodes.size === 1) {
  // replace the current block rather than split the block
  change = change.replaceNodeByKey(editorState.anchorBlock.key, quote);
  }
  