title: _this2.setState is not a function error in react-native
body: <p>I have this code for a drop down menu: </p>
  
  <pre><code>&lt;Picker
      selectedValue= {() =&gt; {this.state.attendance}}
      onValueChange= {(att) =&gt; this.setState({attendance: att})}&gt;
      &lt;Picker.Item label="Present" value="present" /&gt;
      &lt;Picker.Item label="Half Day" value="halfday" /&gt;
      &lt;Picker.Item label="Informed Absent" value="in-absent" /&gt;
      &lt;Picker.Item label="Uninformed Absent" value="un-absent" /&gt;
  &lt;/Picker&gt;
  </code></pre>
  
  <p>But I get an error, "_this2.setState is not a function" on running the app. I know how react-native doesn't auto bind, but from my research the arrow syntax ought to fix it. Anyone knows why this error is still being throw?</p>