ComboBox or Select - Set items in BeforeShow() event

[michele.borgna]

When I open a detail view in edit mode, I have to prepare the available options for a ComboBox or Select field.

public void onBeforeShow(BeforeShowEvent event) {
    // Set available options
    field1.setItems(List.of("A", "B", "C"));
}

I found that using setItems(), the field loses the previously entity value and the ComboBox becomes empty. I need to manually save and restore the value.
Is there a better way to do this?
Am I doing something wrong?
Thank you