title: React Native ref to element is undefined in flatlist item
body: <p>I am trying to get a ref to an element whilst rendering a flatlist but the ref is coming back as undefined. 
  See relevant code below:</p>
  
  <p>Constructor:</p>
  
  <pre><code>constructor(props) {
      super(props);
  
      this.modalRef = createRef();
  }
  </code></pre>
  
  <p>RenderItem:</p>
  
  <pre><code>_renderItem = ({ item }, context) =&gt; {
      const modalRef = context.modalRef.current; // Why is context.modalRef.current null?
      let onPress = () =&gt; alert("Something went wrong...");
  
      return (
          &lt;View style={styles.item}&gt;
              &lt;ListItemCard
                  id={item.id}
                  value="Some value.."
                  onPress={onPress()}
              /&gt;
          &lt;/View&gt;
      );
  };
  </code></pre>
  
  <p>Within Render function:</p>
  
  <p>Modal on screen:</p>
  
  <pre><code>    &lt;Modal
      ref={this.modalRef}
      text="Some text.."
      onPressClose={() =&gt;
          this.modalRef.current.close()
      }
  /&gt;
  </code></pre>
  
  <p>Flatlist:</p>
  
  <pre><code>  &lt;FlatList
      data={data}
      renderItem={item =&gt; this._renderItem(item, this)}
      keyExtractor={(item, index) =&gt; index.toString()}
  /&gt;
  </code></pre>