DevExpress LookUpEditSettings Default Text Property to Display

I'm currently developing Point-of-Sale system.

In "Purchase Order" form, I use LookUpEditSettings in GridControl (for choosing the products LineItem). I use "SearchLookUpEditStyleSettings" for LookUpEditSettings. It's ok and I'm very happy.

But I want to display "Select Product" text in LookUpEditSettings before the user choosing any product (items) from PopupContentTemplate. After the user chose the item, "Select Product" text will be disappeared and selected item will be displayed.

For the above condition, NullText property can be used. But, NullText property only works when the EditValue is null. I want NullText property works when the EditValue is Zero. It's because my DataSource is Integer. As you know, Integer has no null value and it has only Zero value.

(P.S. I know Integer can be "int?" for nullable Integer. But I don't want my "ProductCode" to be nullable Integer.)

Please, help me.