don't call respondWith on empty value

sw.js

@@ -34,28 +34,27 @@ self.addEventListener("install", event => {
   event.waitUntil(initCache())
 })
 
-async function cacheFirst(request) {
+self.addEventListener('fetch', event => {
+  const request = event.request
+  cacheFirst(event.request))
   if (request.url.includes('/-/frame')) {
     const url = new URL(request.url)
     if (url.pathname === '/-/frame') {
       const html = url.searchParams.get('html')
       const csp = url.searchParams.get('csp')
-      return new Response(html, {
+      event.respondWith(new Response(html, {
         headers: {
           'Content-Type': 'text/html; charset=utf-8',
           'Content-Security-Policy': csp,
-        }
-      })
+        },
+      }))
+      return
     }
   }
   const resp = await caches.match(request)
   if (resp) {
-    return resp
+    event.respondWith(resp)
   }
-}
-
-self.addEventListener('fetch', event => {
-  event.respondWith(cacheFirst(event.request))
 })
 
 self.addEventListener('activate', event => {