TypeError: 'NoneType' object is not callable when joining netCDF files

[paul0207]

I'm trying to join multiple netCDF files with the following:

import xarray as xr
combined = xr.open_mfdataset('Burst*.nc',combine="nested", concat_dim="time", group="Data/Burst")
combined.to_netcdf('nc_combined.nc')

When running the lines interactively, everything works, but when running it as a script I get the following error multiple times:
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/xarray/backends/file_manager.py", line 246, in del
TypeError: 'NoneType' object is not callable

Thanks,
Paul