My android app stop I don´t know how get a value of SQLite

I'm new programmer, I need just one value ("IdUser") of SQLite and parse to String variable, but I don't have any idea to catch it and return that value. My app stopped because of this.

Activity.java

public void onClick(View arg0) {

        String LOGIN2 ="Users";
        String eti = loginDataBaseAdapter.CheckEtiqueta(LOGIN2);

        Toast.makeText(getApplicationContext(), eti, Toast.LENGTH_LONG).show();


        }

UsersDatabaseHelper.java

    public String CheckEtiqueta(String LOGIN2) {

        Cursor c = db.rawQuery("SELECT * FROM "+LOGIN2+" WHERE IdUser = 20 ORDER BY IdUser LIMIT1", null);

        while (c.moveToNext()) {
             //Sample operation with the Database
             if (c.getString(c.getColumnIndex("IdUser"))!=null) { 
                   LOGIN2=c.getString(c.getColumnIndex("IdUser"));
                   return LOGIN2;
             }
        }
        return LOGIN2;


    }

Thanks