unavailable airspace data crashes the docking widget

[ReimarBauer]

Fatal error in MSS 7.0.6. on Linux-5.4.0-131-generic-x86_64-with-glibc2.27
Python 3.10.4 | packaged by conda-forge | (main, Mar 24 2022, 17:38:57) [GCC 10.3.0]

Please report bugs in MSS to https://github.com/Open-MSS/MSS

Information about the fatal error:

Traceback (most recent call last):
  File "/home/reimar/miniconda3/envs/mssenv/lib/python3.10/site-packages/mslib/msui/airdata_dockwidget.py", line 101, in redraw_map
    self.view.map.set_draw_airspaces(self.cbDrawAirspaces.isChecked(), self.cbAirspaces.currentData(),
  File "/home/reimar/miniconda3/envs/mssenv/lib/python3.10/site-packages/mslib/msui/mpl_map.py", line 377, in set_draw_airspaces
    self.draw_airspaces(country_codes, range_km)
  File "/home/reimar/miniconda3/envs/mssenv/lib/python3.10/site-packages/mslib/msui/mpl_map.py", line 384, in draw_airspaces
    airspaces = copy.deepcopy(get_airspaces(countries))
  File "/home/reimar/miniconda3/envs/mssenv/lib/python3.10/site-packages/mslib/utils/airdata.py", line 179, in get_airspaces
    for elem in airspace:
TypeError: 'NoneType' object is not iterable