Chained .struct.unnest() PanicException

[cmdlineluser]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl

pl.select(
    pl.lit({"x": {"y": 1}}).struct.unnest().struct.unnest()
)
# PanicException: should be expanded

Log output

thread '<unnamed>' panicked at crates/polars-plan/src/dsl/function_expr/struct_.rs:123:34:
should be expanded

Issue description

Not sure if this is supposed to work or not.

Using .field() first works.

pl.select(
    pl.lit({"x": {"y": 1}}).struct.field("x").struct.unnest()
)

# shape: (1, 1)
# ┌─────┐
# │ y   │
# │ --- │
# │ i64 │
# ╞═════╡
# │ 1   │
# └─────┘

Expected behavior

No panic.

Installed versions

--------Version info---------
Polars:              1.20.0
Index type:          UInt32
Platform:            macOS-13.6.1-arm64-arm-64bit-Mach-O
Python:              3.13.0 (main, Oct  7 2024, 05:02:14) [Clang 15.0.0 (clang-1500.1.0.2.5)]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            0.12.0
fsspec               <not installed>
gevent               <not installed>
google.auth          <not installed>
great_tables         0.14.0
matplotlib           <not installed>
nest_asyncio         <not installed>
numpy                2.1.3
openpyxl             3.1.5
pandas               2.2.3
pyarrow              18.0.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           3.2.0