message: Prevent exception on page unload
  Null check when saving sent history
diff: @@ -74,7 +74,7 @@ export default class MessageComposer extends React.Component {
  }
  onPageUnload(event) {
  - if (this.messageComposerInput) {
  + if (this.messageComposerInput && this.messageComposerInput.sentHistory) {
  this.messageComposerInput.sentHistory.saveLastTextEntry();
  }
  }
  