Version: kwant.__version__ == '1.0.0'
Problem: when kwant.plotter.map() is given the axes to plot on such as
kwant.plotter.plot(system, stuff_to_plot, ax = gca())
kwant/plotter.py first sets fig=None in line 1523 and the tries to create colorbar for a None object in line 1536
Possible solution: Either disable colorbar in this case
line 1535: if colorbar and fig is not None:
Or figure out how to create another axes with colorbar.
Fixed. Until the next release, you can use colorbar=False:
kwant.plotter.plot(system, stuff_to_plot, ax = gca(), colorbar=False)
as a workaround.