fix(darkmode): check for null (#1531)

quartz/components/scripts/darkmode.inline.ts

@@ -27,9 +27,10 @@ document.addEventListener("nav", () => {
 
   // Darkmode toggle
   const themeButton = document.querySelector("#darkmode") as HTMLButtonElement
+  if (themeButton) {
     themeButton.addEventListener("click", switchTheme)
     window.addCleanup(() => themeButton.removeEventListener("click", switchTheme))
-
+  }
   // Listen for changes in prefers-color-scheme
   const colorSchemeMediaQuery = window.matchMedia("(prefers-color-scheme: dark)")
   colorSchemeMediaQuery.addEventListener("change", themeChange)