error in on_pick_event

Don't know how it happen. I think we can just catch and return on this function when this error happens.

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 299, in on_pick_event
    chan_id = ax.chan
AttributeError: 'Axes' object has no attribute 'chan'