按下后退按钮时QR Code阅读器崩溃

Question

阅读器在扫描时工作正常，但是如果我按回去，应用程序将崩溃。

package com.example.vict.tst;
import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
import com.google.zxing.integration.android.IntentIntegrator;
import com.google.zxing.integration.android.IntentResult;

public class MainActivity extends AppCompatActivity {

Button btnScan;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    btnScan = (Button) findViewById(R.id.btnScan);
    final Activity activity = this;

    btnScan.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            IntentIntegrator integrator = new IntentIntegrator(activity);
            integrator.setDesiredBarcodeFormats(IntentIntegrator.QR_CODE_TYPES);
            integrator.setPrompt("SCANNER!");
            integrator.setCameraId(0);
            integrator.initiateScan();

        }
    });
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    IntentResult result = IntentIntegrator.parseActivityResult(requestCode,resultCode,data);
    if (result != null)
    {
        if (result.getContents().equals("HELLO WORLD"))
        {
            alert(result.getContents());
        }else{
            alert("CANCELED");
        }
    }else {
        super.onActivityResult(requestCode, resultCode, data);
    }
}

private void alert(String msg)
{
    Toast.makeText(getApplicationContext(),msg,Toast.LENGTH_LONG).show();
}
}

我已经在finish()中尝试ELSE，但似乎无济于事。该应用程序无故崩溃。 如果扫描成功，则应用程序将继续正常运行。 如果是错误消息，表明QR CODE读取了该应用程序仍然可以正常工作（返回活动以再次按下按钮） 但是，如果我按“后退”按钮，应用程序将关闭。

LOGCAT：

06-26 00:48:39.091 26377-26377/com.example.vict.tst E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.victi.testeinuteis, PID: 26377
    java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=49374, result=0, data=null} to activity {com.example.victi.testeinuteis/com.example.victi.testeinuteis.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
        at android.app.ActivityThread.deliverResults(ActivityThread.java:5007)
        at android.app.ActivityThread.handleSendResult(ActivityThread.java:5050)
        at android.app.ActivityThread.access$1600(ActivityThread.java:230)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1876)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:7409)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
        at com.example.victi.testeinuteis.MainActivity.onActivityResult(MainActivity.java:43)
        at android.app.Activity.dispatchActivityResult(Activity.java:7165)
        at android.app.ActivityThread.deliverResults(ActivityThread.java:5003)
        at android.app.ActivityThread.handleSendResult(ActivityThread.java:5050) 
        at android.app.ActivityThread.access$1600(ActivityThread.java:230) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1876) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:148) 
        at android.app.ActivityThread.main(ActivityThread.java:7409) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)