message: Fix markdown view for React 16
  For some reason, the recent change to React calls the method when the body element is present but the div is not. We need to check to be sure both are present before continuing to avoid an NPE.
diff: @@ -36,7 +36,8 @@ export class SyncMarkdownView extends React.Component<{content: string}, {}> {
  }
  updateHeight() {
  - if (!this.frame || !this.frame.contentWindow.document.body) {
  + if (!this.frame || !this.frame.contentWindow.document.body ||
  + !this.frame.contentWindow.document.body.firstChild) {
  return;
  }
  this.frame.style.height = `${this.frame.contentWindow.document.body.firstChild.offsetHeight + 50}px`;
  