Android Studio i get exception when i try to set data on database

Question

I want to save data to database. But i get null pointer exception from getWritableDatabase(); method. What can i do ?

Add database;

String hisse_adi= hisseListe.get(position).getSymbol();
float hisse_alis= Float.valueOf(String.valueOf(hisseListe.get(position).getClose()));
Float hisse_adet= Float.valueOf(String.valueOf(editTextCuzdanEkleHisse.getText())); 
new CuzdanDaoHisse().HisseEkle(vt,hisse_adi,hisse_alis,hisse_adet);

Database Class

public void HisseEkle(VeriTabani veriTabani,String hisse_adi,Float hisse_alis_fiyati,
                         Float hisse_alis_adedi){
        SQLiteDatabase db = veriTabani.getWritableDatabase();
        ContentValues values = new ContentValues();

        values.put("hisse_adi",hisse_adi);
        values.put("hisse_alis_fiyati",hisse_alis_fiyati);
        values.put("hisse_alis_adedi",hisse_alis_adedi);

        db.insertOrThrow("hisse",null,values);
        db.close();

    }

Exception is ;

java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase com.kdb2018.sasa.percent.veriTabani.VeriTabani.getWritableDatabase()' on a null object reference
        at com.kdb2018.sasa.percent.veriTabani.CuzdanDaoHisse.HisseEkle(CuzdanDaoHisse.java:38)
        at com.kdb2018.sasa.percent.fragment.Fragment_DovizHisse$5.onClick(Fragment_DovizHisse.java:225)