code: def update_cycles(self):
          
          self.idx_cycle.clear()
  
          try:
              self.cycles = self.parent.notes.annot.get_cycles()
  
          except ValueError as err:
              self.idx_cycle.setEnabled(False)
              msg = 'There is a problem with the cycle markers: ' + str(err)
              self.parent.statusBar().showMessage(msg)
  
          else:
              if self.cycles is None:
                  self.idx_cycle.setEnabled(False)
              else:
                  self.idx_cycle.setEnabled(True)
                  for i in range(len(self.cycles)):
                      self.idx_cycle.addItem(str(i+1))
docs: Enable cycles checkbox only if there are cycles marked, with no
          errors.