TypeError calculating sunlight coverage when no pytroll-schedule installed

[gerritholl]

Describe the bug

When I try to calculate sunlight coverage while pytroll-schedule is not installed, I get a TypeError.

To Reproduce

from pyresample import create_area_def
from pyproj import CRS
from trollflow2.plugins import _get_sunlight_coverage
from datetime import datetime
euro4326 = create_area_def(
        "epsg4326europa",
        CRS.from_epsg(4326),
        resolution=1/128,
        area_extent=(-10, 40, 20, 60),
        proj_id="EQC_PLAT")
print(_get_sunlight_coverage(euro4326, datetime(2021, 12, 29, 12)))

Expected behavior

Some number between 0 and 1.

Actual results

Traceback (most recent call last):
  File "/data/gholl/checkouts/protocode/mwe/trollflow2-sunlight-typerror.py", line 11, in <module>
    print(_get_sunlight_coverage(euro4326, datetime(2021, 12, 29, 12)))
  File "/data/gholl/checkouts/trollflow2/trollflow2/plugins/__init__.py", line 666, in _get_sunlight_coverage
    poly = get_twilight_poly(start_time)
TypeError: 'NoneType' object is not callable

Environment Info:

• OS: openSUSE Leap 15.3
• Trollflow2 Version: 0.12.0
• Pyresample Version: 1.22.3

Additional context

Found this when calling _get_sunlight_coverage directly to find out why it gives 0 for me in the middle of the day. Perhaps there will be a new issue soon...