title: Field values are added to collection as null with React Hooks + Firestore
body: <p>I'm working on a React Native app and I have a form I want to post to Firestore using React Hooks. It's almost working, but the field values show up as null in Firebase. What am I doing wrong?
  Using 
  react-native 0.60.5 and
  react-native-firebase 5.6.0. Thanks in advance!</p>
  
  <pre><code>const [ trackTitle, setTitle ] = useState('')
  const [ artistName, setArtist ] = useState('')
  </code></pre>
  
  <pre><code>function onSubmit(e) {
      firestore()
      .collection('tracks')
          .add(
            {
            trackTitle,
            artistName
          })
  }
  </code></pre>
  
  <pre><code>&lt;TextInput 
       value={trackTitle} 
       onChange={e =&gt; setTitle(e.target.value)}
       style={styles.submissionInput}
       multiline={false}
  /&gt; 
  
  &lt;TextInput 
       value={artistName} 
       onChange={e =&gt; setArtist(e.target.value)}
       style={styles.submissionInput}
       multiline={false}
  /&gt; 
  </code></pre>
  
  <pre><code>&lt;TouchableOpacity
       onPress={onSubmit}
       style={styles.navButtons}&gt;
  </code></pre>
  
  <p>Screenshot of the Firebase console:</p>
  
  <p><img src="https://user-images.githubusercontent.com/29167186/79082815-69edd680-7cde-11ea-8b63-1652c811604d.png" alt=""></p>