Android : onbackpressed of my listactvity this error occur,

/*error

java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=1, result=0, data=null} to activity {com.example.mydatabaseapp/com.example.mydatabaseapp.CashRecieved}: java.lang.NullPointerException8"/

//onabackpressed function

public boolean onKeyDown(int keyCode, KeyEvent event) { // TODO Auto-generated method stub

    if(keyCode==KeyEvent.KEYCODE_BACK){
    new AlertDialog.Builder(ShowSAVED.this).setTitle(
    "Attention").setMessage(
    "Want to exit ?")
    .setPositiveButton("YES",
            new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog,
                        int which) {
                    finish();
                }
            }).setNegativeButton("NO",
            new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog,
                        int which) {

                }
            }).show();
    return true;
    }else{
        return false;
    }

    //return super.onKeyDown(keyCode, event);

}