Graph.from_geodataframe() fails without crs

[RKBuck1]

I'm working with some toy data that doesn't have a crs and getting the following error from Graph.geodataframe():

AttributeError                            Traceback (most recent call last)
[<ipython-input-12-0266b8024caa>](https://localhost:8080/#) in <cell line: 1>()
----> 1 graph = Graph.from_geodataframe(zones, reproject = False)

[/usr/local/lib/python3.10/dist-packages/gerrychain/graph/graph.py](https://localhost:8080/#) in from_geodataframe(cls, dataframe, adjacency, cols_to_add, reproject, ignore_errors)
    181 
    182         graph.add_data(df, columns=cols_to_add)
--> 183         graph.graph["crs"] = df.crs.to_json()
    184         return graph
    185 

AttributeError: 'NoneType' object has no attribute 'to_json' 

I can work around it by setting the crs to a pseudo Plate Carrée projection, but it seems like it might make more sense to issue a warning here instead if the GeoDataFrame doesn't already have a crs assigned