message: fix undo on pasting plaintext content
diff: @@ -986,7 +986,11 @@ export default class MessageComposerInput extends React.Component {
  }
  }
  case 'text':
  - return change.insertText(transfer.text);
  + // don't skip/merge so that multiple consecutive pastes can be undone individually
  + return change
  + .setOperationFlag("skip", false)
  + .setOperationFlag("merge", false)
  + .insertText(transfer.text);
  }
  };
  @@ -1538,7 +1542,7 @@ export default class MessageComposerInput extends React.Component {
  let {placeholder} = this.props;
  // XXX: workaround for placeholder being shown when there is a formatting block e.g blockquote but no text
  - if (isEmpty && this.state.editorState.startBlock.type !== DEFAULT_NODE) {
  + if (isEmpty && this.state.editorState.startBlock && this.state.editorState.startBlock.type !== DEFAULT_NODE) {
  placeholder = undefined;
  }
  