service.py: wizard window does not have visible attribute

[MilhouseVH]

If Kodi is shutdown when the First Run Wizard is active the following exception occurs:

2019-11-16 03:24:54.690 T:1929  NOTICE: stop dvd detect media
2019-11-16 03:24:54.692 T:1951   ERROR: EXCEPTION Thrown (PythonToCppException) : -->Python callback/script returned the following error<--
                                             - NOTE: IGNORING THIS CAN LEAD TO MEMORY LEAKS!
                                            Error Type: <class 'AttributeError'>
                                            Error Contents: 'wizard' object has no attribute 'visible'
                                            Traceback (most recent call last):
                                              File "/usr/share/kodi/addons/service.libreelec.settings/service.py", line 98, in <module>
                                                if oe.winOeMain.visible == True:
                                            AttributeError: 'wizard' object has no attribute 'visible'
                                            -->End of Python script error report<--
2019-11-16 03:24:59.693 T:1929   ERROR: CPythonInvoker(0, /usr/share/kodi/addons/service.libreelec.settings/service.py): script didn't stop in 5 seconds - let's kill it
2019-11-16 03:24:59.889 T:1929  NOTICE: Application stopped

Add a visible attribute to the wizard window, and be a bit more defensive for any window that may not have a visible attribute.