title: ToolbarAndroid react native dynamic subtitle bind to state
body: <p>I am trying to set "ToolbarAndroid"'s subtitle based on state variable with no success, </p>
  
  <p>I set the initial state to null and I am trying to do something like "display subtitle only if it isn't null"</p>
  
  <pre><code>  constructor(props) {
      super(props);
  
      this.state = {
        subtitle: null
      }
    }
  
    render() { ...
  
    &lt;ToolbarAndroid
     ..
          subtitle={() =&gt; {
            if (this.state.subtitle !== null) {
             return this.state.subtitle;
            } else {
             return '';
            }
          }}
  </code></pre>
  
  <p>what am I missing?</p>