get_df fails with default arguments

[WeilerP]

Description

Running scvelo.core.get_df (scv.read_load.get_df on scvelo@master) fails when using it default arguments.

import scvelo as scv

adata = scv.datasets.pancreas()
scv.core.get_df(adata)

Error

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-1cfc8708fcb5> in <module>
----> 1 scv.core.get_df(adata)

~/code/scvelo/scvelo/core/_anndata.py in get_df(data, keys, layer, index, columns, sort_values, dropna, precision)
    223 
    224         keys = [keys] if isinstance(keys, str) else keys
--> 225         key = keys[0]
    226 
    227         s_keys = ["obs", "var", "obsm", "varm", "uns", "layers"]

TypeError: 'NoneType' object is not subscriptable

Versions:

scvelo@dev
scvelo==0.2.3 scanpy==1.7.2 anndata==0.7.6 loompy==3.0.6 numpy==1.20.3 scipy==1.6.3 matplotlib==3.4.2 sklearn==0.24.2 pandas==1.2.4