title: Not Retrieved React Native Firestore Subcollection
body: <p>I have a problem when i want to get subcollections.
  Can anybody help me, please ? </p>
  
  <p><strong>firestore structure :</strong> </p>
  
  <pre><code>questions(collection) -&gt;  curretUser.id(doc) -&gt; questions(collection) -&gt; questionIDautoCreated(doc)-&gt; {title: "", description: "" }
  </code></pre>
  
  <p><strong>And i try to get data:</strong></p>
  
  <pre><code>return (dispatch) =&gt; {
    firebase.firestore().collection('questions').doc(currentUser.uid)
    .collection('questions').get()
    .then((documentSnapshot) =&gt; {
      const value = documentSnapshot.data();
      console.log('Value Succeed : ');
      console.log(value);
    });
  };
  </code></pre>
  
  <p><strong>Error :</strong> </p>
  
  <p><strong><em>Possible Unhandled Promise Rejection (id:0)</strong>
  <strong>Type Error: documentSnapshot.data is not a function</em></strong></p>
  
  <p>Thank You</p>