You dont keep combo boxes in your QTableWidget initially.
keep one combo boxe pointer seperatally as a member of class with hide.

whenever you click in any cell you do the following:
1. initialize your combo box with your data.
2. take data of that tableWidget's selected cell data and match with combo box data to make it selected in combobox
3. set combo box item in selected cell and show
4. after editing you read data of combo box and place in cell
5. hide your combo box.