message: null-guard dataset mxTheme to prevent html exports from exploding
diff: @@ -40,7 +40,7 @@ const getExportCSS = async (usedClasses: Set<string>): Promise<string> => {
  // only include bundle.css and the data-mx-theme=light styling
  const stylesheets = Array.from(document.styleSheets).filter(s => {
  return s.href?.endsWith("bundle.css") ||
  - (s.ownerNode as HTMLStyleElement).dataset.mxTheme.toLowerCase() === "light";
  + (s.ownerNode as HTMLStyleElement).dataset.mxTheme?.toLowerCase() === "light";
  });
  let css = "";
  