Can't load static image composite multiple times

[djhoese]

Releated to this (sort of), I've just discovered that reloading a composite that doesn't require any of the actual data from a sensor also fails:

# Example with SEVIRI, substitute whatever data you have
scn = Scene(['D:/sat_data/SEV/MSG3-SEVI-MSG15-0100-NA-20160326144242.057000000Z-NA.nat'], reader='seviri_l1b_native')
scn.load(["_night_background"])
print("LOADED")
scn.load(["_night_background"])
print("LOADED AGAIN")

Expected result: Prints both statements.

Actual result: Prints LOADED and raises this error before reaching second print.

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In [2], line 4
      2 scn.load(["_night_background"])
      3 print("LOADED")
----> 4 scn.load(["_night_background"])
      5 print("LOADED AGAIN")

File ~\PycharmProjects\satpy\satpy\scene.py:1346, in Scene.load(self, wishlist, calibration, resolution, polarization, level, generate, unload, **kwargs)
   1341 needed_datasets = (self._wishlist | dataset_keys) - set(self._datasets.keys())
   1342 query = DataQuery(calibration=calibration,
   1343                   polarization=polarization,
   1344                   resolution=resolution,
   1345                   level=level)
-> 1346 self._update_dependency_tree(needed_datasets, query)
   1348 self._wishlist |= needed_datasets
   1350 self._read_datasets_from_storage(**kwargs)

File ~\PycharmProjects\satpy\satpy\scene.py:1356, in Scene._update_dependency_tree(self, needed_datasets, query)
   1354 def _update_dependency_tree(self, needed_datasets, query):
   1355     try:
-> 1356         comps, mods = load_compositor_configs_for_sensors(self.sensor_names)
   1357         self._dependency_tree.update_compositors_and_modifiers(comps, mods)
   1358         self._dependency_tree.populate_with_keys(needed_datasets, query)

File ~\PycharmProjects\satpy\satpy\scene.py:170, in Scene.sensor_names(self)
    158 @property
    159 def sensor_names(self) -> set[str]:
    160     """Return sensor names for the data currently contained in this Scene.
    161 
    162     Sensor information is collected from data contained in the Scene
   (...)
    168 
    169     """
--> 170     contained_sensor_names = self._contained_sensor_names()
    171     reader_sensor_names = set([sensor for reader_instance in self._readers.values()
    172                                for sensor in reader_instance.sensor_names])
    173     return contained_sensor_names | reader_sensor_names

File ~\PycharmProjects\satpy\satpy\scene.py:185, in Scene._contained_sensor_names(self)
    183         sensor_names.update(data_arr.attrs["sensor"])
    184     else:
--> 185         raise TypeError("Unexpected type in sensor collection")
    186 return sensor_names

TypeError: Unexpected type in sensor collection

Originally posted by @simonrp84 in #2242 (comment)

[djhoese]

This is actually worse than we originally thought. Any static composite doesn't allow you to load anything afterward:

In [1]: from satpy import Scene; from glob import glob

In [2]: scn = Scene(reader='abi_l1b', filenames=glob('/data/satellite/abi/20181127/*.nc'))

In [3]: scn.load(['_night_background'])
cannot convert float NaN to integer

In [4]: scn.load(['C01'])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In [4], line 1
----> 1 scn.load(['C01'])

File ~/repos/git/satpy/satpy/scene.py:1350, in Scene.load(self, wishlist, calibration, resolution, polarization, level, modifiers, generate, unload, **kwargs)
   1344 needed_datasets = (self._wishlist | dataset_keys) - set(self._datasets.keys())
   1345 query = DataQuery(calibration=calibration,
   1346                   polarization=polarization,
   1347                   resolution=resolution,
   1348                   modifiers=modifiers,
   1349                   level=level)
-> 1350 self._update_dependency_tree(needed_datasets, query)
   1352 self._wishlist |= needed_datasets
   1354 self._read_datasets_from_storage(**kwargs)

File ~/repos/git/satpy/satpy/scene.py:1360, in Scene._update_dependency_tree(self, needed_datasets, query)
   1358 def _update_dependency_tree(self, needed_datasets, query):
   1359     try:
-> 1360         comps, mods = load_compositor_configs_for_sensors(self.sensor_names)
   1361         self._dependency_tree.update_compositors_and_modifiers(comps, mods)
   1362         self._dependency_tree.populate_with_keys(needed_datasets, query)

File ~/repos/git/satpy/satpy/scene.py:170, in Scene.sensor_names(self)
    158 @property
    159 def sensor_names(self) -> set[str]:
    160     """Return sensor names for the data currently contained in this Scene.
    161
    162     Sensor information is collected from data contained in the Scene
   (...)
    168
    169     """
--> 170     contained_sensor_names = self._contained_sensor_names()
    171     reader_sensor_names = set([sensor for reader_instance in self._readers.values()
    172                                for sensor in reader_instance.sensor_names])
    173     return contained_sensor_names | reader_sensor_names

File ~/repos/git/satpy/satpy/scene.py:185, in Scene._contained_sensor_names(self)
    183         sensor_names.update(data_arr.attrs["sensor"])
    184     else:
--> 185         raise TypeError("Unexpected type in sensor collection")
    186 return sensor_names

TypeError: Unexpected type in sensor collection