title: Reading QR code crashes iOS(Objective C) application
body: <p>I have created one React Native application for barcode/QRCode scanning functionality. I used third party barcode scanner library.I am able to scan barcode properly, but application getting crashed on QR code scan.Here is sample code:</p>
  
  <pre><code>- (void)captureOutput:(AVCaptureOutput *)output didOutputMetadataObjects:(NSArray&lt;__kindof AVMetadataObject *&gt; *)metadataObjects fromConnection:(AVCaptureConnection *)connection
  {
   for (AVMetadataObject *object in metadataObjects) {
    if ([object isKindOfClass:[AVMetadataMachineReadableCodeObject class]]) {
      if (_callback != nil) {
          _callback(@{
                      @"format": object.type,
                      @"content": [(AVMetadataMachineReadableCodeObject *)object stringValue]
                      });
      }
  }
  </code></pre>
  
  <p>}
  }</p>
  
  <p>it returns nil when I am trying scan QR code and application is getting crash on this line</p>
  
  <pre><code>[(AVMetadataMachineReadableCodeObject *)object stringValue] 
  </code></pre>
  
  <p>and getting exception error is </p>
  
  <pre><code>-[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[0]
  </code></pre>