Using wradli's function "polar.spherical_to_proj" throws an "AttributeError: 'NoneType' object has no attribute 'TransformPoints'" error?

Question

i'm trying to plot the radar scan strategy according to this example :

https://docs.wradlib.org/projects/old-docs/en/1.9.0/notebooks/visualisation/wradlib_plot_scan_strategy.html

When i try to run the wradlib.georef.polar.spherical_to_proj function i get an AttributeError: 'NoneType' object has no attribute 'TransformPoints'even when i try to the example of the documentation as mentioned there.

Basically i am at this point:

nrays = 360
nbins = 416
range_res = 250

coord = wrl.georef.sweep_centroids(nrays, range_res, nbins, 1)  # slant range, azimuth, elevation
coords = wrl.georef.polar.spherical_to_proj(coord[..., 0], np.degrees(coord[..., 1]), coord[..., 2], site)
lon = coords[..., 0]
lat = coords[..., 1]
alt = coords[..., 2]