Application closing in user interaction after being summarized

I'm playing around with an application that controls my mobile's flash , but it does force close as soon as the user interacts with the app after it has been summarized .

For example:

My code:

package com.bravosix.lanterna;

import android.app.Activity;
import android.content.pm.ActivityInfo;
import android.content.pm.PackageManager;
import android.hardware.Camera;
import android.hardware.Camera.Parameters;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageButton;
import android.widget.Toast;

public class Tela_Lanterna extends Activity {

    private boolean isFlashOn;
    private Camera mCamera;
    private ImageButton mImageButton;
    private PackageManager mPackage = null;
    // private AlertDialog mNoCameraAlert;
    private Parameters mParameters;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
        setContentView(R.layout.layout_lanterna);

        mPackage = this.getPackageManager();
        mImageButton = (ImageButton) findViewById(R.id.toggle_img);
        mImageButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                toggleFlash();
            }
        });

        mCamera = Camera.open();
        mParameters = mCamera.getParameters();

        if (!verificarFlash(mPackage)) {
            Toast.makeText(this, "Dispositivo não possui flash!",
                    Toast.LENGTH_SHORT).show();
            return;
        }

    }

    private void toggleFlash() {
        if (!isFlashOn) {
            mParameters = mCamera.getParameters();
            mParameters.setFlashMode(Parameters.FLASH_MODE_TORCH);
            mCamera.setParameters(mParameters);
            mCamera.startPreview();
            mImageButton.setImageResource(R.drawable.btn_ligado);
            isFlashOn = true;
        } else if (isFlashOn) {
            mParameters = mCamera.getParameters();
            mParameters.setFlashMode(Parameters.FLASH_MODE_OFF);
            mCamera.setParameters(mParameters);
            mCamera.stopPreview();
            mImageButton.setImageResource(R.drawable.btn_desligado);
            isFlashOn = false;
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        if (isFlashOn) {
            toggleFlash();
        }
    }

    @Override
    protected void onStop() {
        super.onStop();
        if (mCamera != null) {
            mCamera.release();
            mCamera = null;
        }
    }

    private boolean verificarFlash(PackageManager mPackage) {
        if (mPackage.hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH)) {
            return true;
        } else {
            return false;
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_lanterna, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.menu_configs) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

My logcat:

04-22 15:46:47.994: E/AndroidRuntime(29553): FATAL EXCEPTION: main
04-22 15:46:47.994: E/AndroidRuntime(29553): Process: com.bravosix.lanterna, PID: 29553
04-22 15:46:47.994: E/AndroidRuntime(29553): java.lang.NullPointerException
04-22 15:46:47.994: E/AndroidRuntime(29553):    at com.bravosix.lanterna.Tela_Lanterna.toggleFlash(Tela_Lanterna.java:52)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at com.bravosix.lanterna.Tela_Lanterna.access$0(Tela_Lanterna.java:50)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at com.bravosix.lanterna.Tela_Lanterna$1.onClick(Tela_Lanterna.java:35)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at android.view.View.performClick(View.java:4456)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at android.view.View$PerformClick.run(View.java:18462)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at android.os.Handler.handleCallback(Handler.java:733)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at android.os.Handler.dispatchMessage(Handler.java:95)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at android.os.Looper.loop(Looper.java:136)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at android.app.ActivityThread.main(ActivityThread.java:5102)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at java.lang.reflect.Method.invokeNative(Native Method)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at java.lang.reflect.Method.invoke(Method.java:515)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-22 15:46:47.994: E/AndroidRuntime(29553):    at dalvik.system.NativeStart.main(Native Method)

I did not understand why the error was NullPointerException .