Select BRF period bug.

[jnsebgosselin]

What steps will reproduce the problem?

1. Click on the button to select the BRF period
2. Click only ONE time on the graph
3. Click on the button to select the BRF period again (to cancel it)
4. Click on the button to select the BRF period
5. Click on the graph to select the BRF period

Please provide any additional information below

Traceback (most recent call last):
  File "C:\Python36\lib\site-packages\matplotlib\backends\backend_qt5.py", line
289, in mousePressEvent
    guiEvent=event)
  File "C:\Python36\lib\site-packages\matplotlib\backend_bases.py", line 1917, i
n button_press_event
    self.callbacks.process(s, mouseevent)
  File "C:\Python36\lib\site-packages\matplotlib\cbook.py", line 554, in process

    proxy(*args, **kwargs)
  File "C:\Python36\lib\site-packages\matplotlib\cbook.py", line 416, in __call_
_
    return mtd(*args, **kwargs)
  File "C:\Users\jsgosselin\GWHAT\gwhat\HydroCalc2.py", line 1350, in onclick
    self.config_brf.set_datarange(self.brfperiod)
  File "C:\Users\jsgosselin\GWHAT\gwhat\brf_mod\kgs_gui.py", line 302, in set_da
tarange
    date_start = xldate_as_tuple(times[0], 0)
  File "C:\Python36\lib\site-packages\xlrd\xldate.py", line 95, in xldate_as_tup
le
    if xldate < 0.00:
TypeError: '<' not supported between instances of 'NoneType' and 'float'