Method called after release Exception Camera preview

I have one activity class(CameraActivity) which is using my CameraPreview class. In "OnResume" the camera and preview are initiated. In "OnPause", i am releasing camera resources. When application is started, everything works fine inside the "OnResume" but when i start another activity through intent (E.g. open url in browser) and then come back to my activity then exception is occured inside "OnResume" originating CamerPreview class. Please find below the code:

//CameraActivity class

public void onResume(){
    super.onResume();
    Log.d("inside onResume, camera==="+mCamera, "inside onResume");
    try {
        if(mCamera==null)
        {

    setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);


        autoFocusHandler = new Handler();
        mCamera = getCameraInstance();
        int rotation = this.getWindowManager().getDefaultDisplay().getRotation();


        scanner = new ImageScanner();
        scanner.setConfig(0, Config.X_DENSITY, 3);
        scanner.setConfig(0, Config.Y_DENSITY, 3);

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

        FrameLayout preview = (FrameLayout)findViewById(R.id.cameraPreview);
        preview.addView(mPreview);



    }





} catch (Exception e) {
    // TODO Auto-generated catch block
    Log.e("onResume",Log.getStackTraceString(e));
}



public void onPause{
    try {
        super.onPause();
        if (mCamera != null) {
            previewing = false;
            mCamera.stopPreview();
                mCamera.setPreviewCallback(null);
                mCamera.release();
                mCamera = null;
                mPreview=null;


    }
} catch (Exception e) {
    // TODO Auto-generated catch block
    Log.e("releaseCamera",Log.getStackTraceString(e));
}
}



 public static Camera getCameraInstance(){
        Camera c = null;
        try {
            c = Camera.open();
        } catch (Exception e){
            Log.e("getCa