crash in remote code

[jhalmen]

when using both a button and a slider to control the same parameter
if you're using the slider after pressing the button the gui crashes on this line:

tool-pywisp/pywisp/gui.py

Line 1599 in e15b50b

widget.label.setText(widget.widgetName + ": {:.3f}".format(value))

with the following trace:

Traceback (most recent call last):
  File "//pywisp/pywisp/gui.py", line 1592, in <lambda>
    wid.valueChanged.connect(lambda value: self.remoteSliderSendParameter(wid, value))
  File "//pywisp/pywisp/gui.py", line 1574, in remoteSliderSendParameter
    self.remoteSliderUpdate(widget, value)
  File "//pywisp/pywisp/gui.py", line 1599, in remoteSliderUpdate
    widget.label.setText(widget.widgetName + ": {:.3f}".format(value))
AttributeError: 'NoneType' object has no attribute 'setText'

the widget seems to be set to MovablePushButton

pywisp doesn't even have to be connected for this crash to occur.