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.