why is my emulator showing an error: "There was an error accessing your devices external storage"

public static final String TAG = MainActivity.class.getSimpleName();

public static final int TAKE_PHOTO_REQUEST = 0;
public static final int TAKE_VIDEO_REQUEST = 1;
public static final int PICK_PICTURE_REQUEST = 2;
public static final int PICK_VIDEO_REQUEST = 3;

public static final int MEDIA_TYPE_IMAGE = 4;
public static final int MEDIA_TYPE_VIDEO = 5;

protected Uri mMediaUri;

protected DialogInterface.OnClickListener mDialogListener = new DialogInterface.OnClickListener() {

    @Override
    public void onClick(DialogInterface dialog, int which) {
        switch(which){
        case 0: //Take Picture
            Intent takePhotoIntent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
            mMediaUri = getOutputMediaFileUri(MEDIA_TYPE_IMAGE);
            if (mMediaUri == null){
                //display an error
                Toast.makeText(MainActivity.this,R.string.error_external_storage,Toast.LENGTH_LONG).show();
            }
            else{
            takePhotoIntent.putExtra(MediaStore.EXTRA_OUTPUT, mMediaUri);
            startActivityForResult(takePhotoIntent, TAKE_PHOTO_REQUEST);
            }
            break;
        case 1: //Take Video
            break;
        case 2: //Choose Picture
            break;
        case 3: //Choose Video
            break;
        }

    }

    private Uri getOutputMediaFileUri(int mediaType) {
        if (isExternalStorageAvailable()){
            //get the URI
            return null;
        }
        else{
        return null;
    }
    }
    private boolean isExternalStorageAvailable(){
        String state = Environment.getExternalStorageState();

        if (state.equals(Environment.MEDIA_MOUNTED)){
            return true;
        }
        else{
            return false;
        }
    }
};

/**
 * The {@link android.support.v4.view.PagerAdapter} that will provide
 * fragments for each of the sections. We use a
 * {@link FragmentPagerAdapter} derivative, which will keep every
 * loaded fragment in memory. If this becomes too memory intensive, it
 * may be best to switch to a
 * {@link android.support.v13.app.FragmentStatePagerAdapter}.
 */
SectionsPagerAdapter mSectionsPagerAdapter;

/**
 * The {@link ViewPager} that will host the section contents.
 */
ViewPager mViewPager;

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

    ParseAnalytics.trackAppOpened(getIntent());

    ParseUser currentUser = ParseUser.getCurrentUser();
   if (currentUser == null) {
        navigateToLogin();
    }
   else{
       Log.i(TAG, currentUser.getUsername());
   }

    // Set up the action bar.
    final ActionBar actionBar = getActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    // Create the adapter that will return a fragment for each of the three
    // primary sections of the activity.
    mSectionsPagerAdapter = new SectionsPagerAdapter (this, getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    // When swiping between different sections, select the corresponding
    // tab. We can also use ActionBar.Tab#select() to do this if we have
    // a reference to the Tab.
    mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
        }
    });

    // For each of the sections in the app, add a tab to the action bar.
    for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
        // Create a tab with text corresponding to the page title defined by
        // the adapter. Also specify this Activity object, which implements
        // the TabListener interface, as the callback (listener) for when
        // this tab is selected.
        actionBar.addTab(
                actionBar.newTab()
                        .setText(mSectionsPagerAdapter.getPageTitle(i))
                        .setTabListener(this));
    }
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (resultCode == RESULT_OK){
        Intent mediaScanIntent = new Intent(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE);
        mediaScanIntent.setData(mMediaUri);
        sendBroadcast(mediaScanIntent);

    }
    else if (resultCode != RESULT_CANCELED){
        Toast.makeText(this,R.string.general_error, Toast.LENGTH_LONG).show();
    }
}


private void navigateToLogin() {
    Intent intent = new Intent (this, LoginActivity.class);
    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
    startActivity(intent);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;

}

@Override
public boolean onOptionsItemSelected(MenuItem item) {

    int itemId = item.getItemId();

    switch (itemId){
    case R.id.action_logout:
        ParseUser.logOut();
        navigateToLogin();
    case R.id.action_edit_friends:
        Intent intent = new Intent(this, EditFriendsActivity.class);
        startActivity(intent);
        case R.id.action_camera:
            AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setItems(R.array.camera_choices, mDialogListener);
            AlertDialog dialog = builder.create();
            dialog.show();


    }
    return super.onOptionsItemSelected(item);
}

@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    // When the given tab is selected, switch to the corresponding page in
    // the ViewPager.
    mViewPager.setCurrentItem(tab.getPosition());
}

@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}

@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}