issues: 2090281639

What happened?

With the 2024.1 release, I am unable to write objects to netCDF after having stacked dimensions with .stack() and called .reset_index() to get rid of the multi-index

What did you expect to happen?

No response

Minimal Complete Verifiable Example

Python import numpy as np import xarray as xr da = xr.DataArray(np.zeros([2, 3]), dims=["x", "y"]) da = da.stack(point=("x", "y")) da = da.reset_index("point") da.to_netcdf("test.nc")

MVCE confirmation

• [X] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [X] Complete example — the example is self-contained, including all data and the text of any traceback.
• [X] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [X] New issue — a search of GitHub Issues suggests this is not a duplicate.
• [X] Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

```Python 86 def ensure_not_multiindex(var: Variable, name: T_Name = None) -> None: 87 if isinstance(var._data, indexing.PandasMultiIndexingAdapter): ---> 88 raise NotImplementedError( 89 f"variable {name!r} is a MultiIndex, which cannot yet be " 90 "serialized. Instead, either use reset_index() " 91 "to convert MultiIndex levels into coordinate variables instead " 92 "or use https://cf-xarray.readthedocs.io/en/latest/coding.html." 93 )

NotImplementedError: variable 'x' is a MultiIndex, which cannot yet be serialized. Instead, either use reset_index() to convert MultiIndex levels into coordinate variables instead or use https://cf-xarray.readthedocs.io/en/latest/coding.html. ```

Anything else we need to know?

Creating the stacked object from scratch and saving it to netCDF works fine. The difference is that type(da.x.variable._data) is xarray.core.indexing.PandasMultiIndexingAdapter if it was stacked and reset and numpy.ndarray if it's created from scratch

Environment