ctrl/cmd click on waveform give error

When ctrl/cmd + click on waveform, a ruler is supposed to show up, but error happens instead.

Traceback (most recent call last):
  File "/Users/ldam/opt/anaconda3/envs/soh/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 270, in process
    func(*args, **kwargs)
  File "/Users/ldam/Documents/GIT/soh_gar/sohstationviewer/sohstationviewer/view/plotting/plotting_widget/plotting_widget.py", line 396, in on_button_press_event
    w.on_ctrl_cmd_click(xdata)
  File "/Users/ldam/Documents/GIT/soh_gar/sohstationviewer/sohstationviewer/view/plotting/plotting_widget/plotting_widget.py", line 420, in on_ctrl_cmd_click
    ruler_text_content = format_time(xdata, self.parent.date_format,
AttributeError: 'WaveformDialog' object has no attribute 'date_format'