Camera view not relesing on button's click listener

I am trying to relese camera on imageview's setonclick listener,but when i run the app and click on imageview my app got crash and shows..

java.lang.runtimeexception method called after release()

I dont know what is issue and why i am not able to intent activities whenever i want,and my app got crash,thanks in advance,

MainMenu.java

 rl_scan.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            Intent intent = new Intent(MainMenu.this, ScanDispatch.class);
            startActivity(intent);
            finish();

        }
    });

ScanDispatch.java

 public class ScanDispatch extends Activity {

// static final String ACTION_SCAN = "com.google.zxing.client.android.SCAN";
// private ImageButton btn_scan;
private TextView current_office_name;

// scanning
private Camera mCamera;
private CameraPreview mPreview;
private Handler autoFocusHandler;
ImageScanner scanner;
boolean barcodeScanned = false;
boolean previewing = true;
// private DrawerLayout mDrawerLayout;
ImageView img_navigation_link;

private FrameLayout preview;

public ScanDispatch() {
}



@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_scan_dispatch);

    consts.pref = getSharedPreferences("pref", MODE_PRIVATE);
    consts.editor = consts.pref.edit();

    current_office_name = (TextView) findViewById(R.id.current_office_name);
    current_office_name.setText(consts.pref.getString("office_name", ""));
    // btn_scan = (ImageButton) findViewById(R.id.btn_scan);
    img_navigation_link = (ImageView) findViewById(R.id.img_navigation_link);

    // scanner

    setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    autoFocusHandler = new Handler();
    mCamera = getCameraInstance();
    scanner = new ImageScanner();
    scanner.setConfig(0, Config.X_DENSITY, 3);
    scanner.setConfig(0, Config.Y_DENSITY, 3);

    mPreview = new CameraPreview(this, mCamera, previewCb, autoFocusCB);

    try {

        if (barcodeScanned) {
            barcodeScanned = false;
            // scanText.setText("Scanning...");
            mCamera.setPreviewCallback(previewCb);
            mCamera.startPreview();
            previewing = true;
            mCamera.autoFocus(autoFocusCB);
        }

    } catch (ActivityNotFoundException anfe) {
        anfe.printStackTrace();

    }

    preview = (FrameLayout) findViewById(R.id.relativeLayout1);

     preview.addView(mPreview);

    img_navigation_link.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {

            mCamera.release();
            preview.removeView(mPreview);
            Intent intent = new Intent(ScanDispatch.this, MainMenu.class);
            startActivity(intent);

        }
    });


}







@SuppressWarnings("deprecation")
public static Camera getCameraInstance() {
    Camera c = null;
    try {
        c = Camera.open();
    } catch (Exception e) {
    }
    return c;
}

static {
    System.loadLibrary("iconv");
}

private Runnable doAutoFocus = new Runnable() {
    @SuppressWarnings("deprecation")
    public void run() {
        if (previewing)
            mCamera.autoFocus(autoFocusCB);
    }
};

// Mimic continuous auto-focusing
@SuppressWarnings("deprecation")
AutoFocusCallback autoFocusCB = new AutoFocusCallback() {
    public void onAutoFocus(boolean success, Camera camera) {
        autoFocusHandler.postDelayed(doAutoFocus, 1000);
    }
};

@SuppressWarnings("deprecation")
PreviewCallback previewCb = new PreviewCallback() {
    private String degd;

    public void onPreviewFrame(byte[] data, Camera camera) {
        Camera.Parameters parameters = camera.getParameters();
        Size size = parameters.getPreviewSize();

        Image barcode = new Image(size.width, size.height, "Y800");
        barcode.setData(data);

        int result = scanner.scanImage(barcode);

        if (result != 0) {
            previewing = false;
            mCamera.setPreviewCallback(null);
            mCamera.stopPreview();

            SymbolSet syms = scanner.getResults();
            for (Symbol sym : syms) {
                // scanText.setText("barcode result " + sym.getData());
                Log.d("ooo", "Barcode result:" + sym.getData());
                String contents = sym.getData();
                consts.editor.putString("qrcode", contents);
                consts.editor.commit();

                barcodeScanned = true;

                Intent intent = new Intent(ScanDispatch.this, DispatchTracking.class);
                intent.putExtra("arrow_val", "1");
                startActivity(intent);
                mCamera.release();
                ScanDispatch.this.finish();



            }
        }
    }
};

CameraPreview.java

 public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {
private SurfaceHolder mHolder;
private Camera mCamera;
private PreviewCallback previewCallback;
private AutoFocusCallback autoFocusCallback;

public CameraPreview(Context context, Camera camera,
                     PreviewCallback previewCb,
                     AutoFocusCallback autoFocusCb) {
    super(context);
    mCamera = camera;
    previewCallback = previewCb;
    autoFocusCallback = autoFocusCb;

    /* 
     * Set camera to continuous focus if supported, otherwise use
     * software auto-focus. Only works for API level >=9.
     */
    /*
    Camera.Parameters parameters = camera.getParameters();
    for (String f : parameters.getSupportedFocusModes()) {
        if (f == Parameters.FOCUS_MODE_CONTINUOUS_PICTURE) {
            mCamera.setFocusMode(Parameters.FOCUS_MODE_CONTINUOUS_PICTURE);
            autoFocusCallback = null;
            break;
        }
    }
    */

    // Install a SurfaceHolder.Callback so we get notified when the
    // underlying surface is created and destroyed.
    mHolder = getHolder();
    mHolder.addCallback(this);

    // deprecated setting, but required on Android versions prior to 3.0
    mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
}

@SuppressWarnings("deprecation")
public void surfaceCreated(SurfaceHolder holder) {
    // The Surface has been created, now tell the camera where to draw the preview.
    try {
        mCamera.setPreviewDisplay(holder);
    } catch (IOException e) {
        Log.d("DBG", "Error setting camera preview: " + e.getMessage());
    }
}

public void surfaceDestroyed(SurfaceHolder holder) {
    // Camera preview released in activity

    /*this.getHolder().removeCallback(this);
    mCamera.stopPreview();
    mCamera.release();*/
    //mCamera.release();
}

public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
    /*
     * If your preview can change or rotate, take care of those events here.
     * Make sure to stop the preview before resizing or reformatting it.
     */
    if (mHolder.getSurface() == null){
      // preview surface does not exist
      return;
    }

    // stop preview before making changes
    try {
        mCamera.stopPreview();
    } catch (Exception e){
      // ignore: tried to stop a non-existent preview
    }

    try {
        // Hard code camera surface rotation 90 degs to match Activity view in portrait
        mCamera.setDisplayOrientation(90);

        mCamera.setPreviewDisplay(mHolder);
        mCamera.setPreviewCallback(previewCallback);
        mCamera.startPreview();
        mCamera.autoFocus(autoFocusCallback);
    } catch (Exception e){
        Log.d("DBG", "Error starting camera preview: " + e.getMessage());
    }
}

}