You should not call the gtk:combo-box-model function or attempt to pack more cells into this combo box via its gtk:cell-layout interface.
GtkComboBoxText as GtkBuildable
The gtk:combo-box-text implementation of the gtk:buildable interface supports adding items directly using the <items> element and specifying <item> elements for each item. Each <item> element supports the regular translation attributes "translatable", "context" and "comments".
The gtk:combo-box-text implementation has a single CSS node with name combobox. It adds the .combo style class to the main CSS nodes of its entry and button children, and the .linked class to the
node of its internal box.