Just a quick update here is the code to populate a combo box

ui_combo_box = qtui_widget.findChild(PySide2.QtWidgets.QComboBox, "comboBox")

get_screen_objects_combo_box(ui_combo_box)

def get_screen_objects_combo_box():
    #-----Get all props -----
    prop_objects = RLPy.RScene.FindObjects( RLPy.EObjectType_Prop )      
    for item in prop_objects:
        ui_combo_box.addItem(item.GetName())

And to get the selected item
ui_combo_box.activated[str].connect(run_combo_box)

def run_combo_box(text):
    print("Combo Box Text: {}".format(text.GetName())

Hope this helps
Chris.