code: def add_text(self, text):
          
          index = self.findText(text)
          while index != -1:
              self.removeItem(index)
              index = self.findText(text)
          self.insertItem(0, text)
          index = self.findText('')
          if index != -1:
              self.removeItem(index)
              self.insertItem(0, '')
              if text != '':
                  self.setCurrentIndex(1)
              else:
                  self.setCurrentIndex(0)
          else:
              self.setCurrentIndex(0)
docs: Add text to combo box: add a new item if text is not found in
          combo box items.