title: Firebase response is not valid JSON object
body: <p>I'm building a React Native App with firebase and over night I got error saying: <code>response is not valid JSON object</code> <br/>
  Since I changed absolutely nothing and it worked just before I went to sleep I have seriously no idea what happend.</p>
  <p>In my code it looks like this:</p>
  <pre><code>const getPosts = FireApp.functions().httpsCallable(&quot;getPosts&quot;);
  
  let res = await getPosts();
  </code></pre>
  <p>I checked the function response before it is send, but it is just JSON</p>
  <pre><code>let response = JSON.stringify(items);
  return response;
  </code></pre>