safety coalesce

src/components/ThemeSwitcher.tsx

@@ -36,7 +36,7 @@ export default function ThemeSwitcher({
       onClick={toggleTheme}
       aria-description={'Change current theme'}
     >
-      <span data-label className="transition-all block absolute w-full mb-auto mt-auto ml-0 mr-0 text-xs left-0 -top-5 opacity-0 rounded-lg">{firstCharOfPreference.toLocaleUpperCase() + restOfPreference.join("")}</span>
+      <span data-label className="transition-all block absolute w-full mb-auto mt-auto ml-0 mr-0 text-xs left-0 -top-5 opacity-0 rounded-lg">{firstCharOfPreference.toLocaleUpperCase() + (restOfPreference ?? []).join("")}</span>
       {themeIcons[preference]}
     </button>
   );