SignalCut add_obs tries to append to a numpy array

While trying to run MLmapmaker with the soconda environment on NERSC I got the following error:

TOAST ERROR: Proc 0: Traceback (most recent call last):
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/toast/mpi.py", line 515, in exception_guard
    yield
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/sotodlib/toast/workflows/toast_so_map.py", line 694, in cli
    main()
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/sotodlib/toast/workflows/toast_so_map.py", line 680, in main
    reduce_data(job, args, data)
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/sotodlib/toast/workflows/toast_so_map.py", line 548, in reduce_data
    ops.mlmapmaker.apply(data)
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/toast/timing.py", line 107, in df
    return f(*args, **kwargs)
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/toast/ops/operator.py", line 107, in apply
    self.exec(data, detectors, use_accel=use_accel, **kwargs)
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/toast/timing.py", line 107, in df
    return f(*args, **kwargs)
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/toast/ops/operator.py", line 47, in exec
    self._exec(
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/toast/timing.py", line 81, in df
    return f(*args, **kwargs)
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/sotodlib/toast/ops/mlmapmaker.py", line 419, in _exec
    self._mapmaker.add_obs(
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/sotodlib/mapmaking.py", line 80, in add_obs
    signal.add_obs(id, obs, nmat, tod)
Proc 0:   File "/global/common/software/sobs/perlmutter/conda_envs/soconda_20230928_0.0.5/lib/python3.10/site-packages/sotodlib/mapmaking.py", line 336, in add_obs
    self.rhs.append(obs_rhs)
Proc 0: AttributeError: 'numpy.ndarray' object has no attribute 'append'

In SignalCut class, self.rhs is a list initially (line 293), and changes to a numpy array in method prepare (line 320).

Read SignalCut in sotodlib/mapmaking/ml_mapmaker.py, especially its prepare and add_obs methods at the referenced lines. Reproduce the MLmapmaker workflow that triggers the traceback, then verify that add_obs completes without the numpy.ndarray append error.