title: ZXing QR code scanner embedded pressing back button during scan issue
body: <p>I have the following scenario :
  I used the Maven repository from Gradle to integrate ZXing into my Android app.</p>
  
  <p>In my scan activity, the code looks like this :</p>
  
  <pre><code>@Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.scan_layout);
  
      IntentIntegrator integrator = new IntentIntegrator(this);
      integrator.setDesiredBarcodeFormats(IntentIntegrator.QR_CODE_TYPES);
      integrator.setPrompt(" ");
      integrator.setScanningRectangle(700, 700);
      integrator.setResultDisplayDuration(0);
      integrator.setCameraId(0);  // Use a specific camera of the device
      integrator.initiateScan();
  }
  
    public void onActivityResult(int requestCode, int resultCode, Intent intent) {
  
      //retrieve scan result
      IntentResult scanningResult = null;
      scanningResult = IntentIntegrator.parseActivityResult(requestCode, resultCode, intent);
  
      if (scanningResult != null) {
       //we have a result
          String scanContent = scanningResult.getContents();
  
          if(isConnected()) {
              requestdata("http://rm360project-001-site1.smarterasp.net/api/endpoint", scanContent);
          }else {
              Toast.makeText(this, "Internet Connection not available", Toast.LENGTH_LONG).show();
          }
      } else {
          Intent getMainScreen = new Intent(ScanScreen.this, MainActivity.class);//pentru test, de sters
          startActivity(getMainScreen);
      }
  }
  </code></pre>
  
  <p>The way I want it to work :<br>
   1. If I scan a QR code, call the function requestdata<br>
   2. If I press back during scan, go to MainActivity</p>
  
  <p>The problem : 
  Even when I press back on my device, the function requestdata is called, I think because scaningResult is never null. Shouldn't it be null when back is pressed?
  Do you have any ideea why this happens?
  Thank you!</p>