How to set dispaly text for ComboBox (checkable) in Qt

I follow these to create checkable combobox

ComboBox of CheckBoxes?

http://ift.tt/2jtuUP4

However when I do a this->Model->clear() then add items, the combobox text (the text combobox displays before user clicking anything) goes blank. The items will still show and are checkable when click on the combobox. I suspect the clear() remove the header and causes this, however I try setHorizontalHeaderLabels etc but I still can't set the combobox text. What am I missing?