Fix butterfly index

[drammock]

fixes bug introduced in #10087

[larsoner]

LGTM, let's merge once CIs come back happy!

[larsoner]

(it would be nice to have a unit test that would have caught this if you can add one without too much effort)

[drammock]

(it would be nice to have a unit test that would have caught this if you can add one without too much effort)

I played around a bit, but actually can't reproduce the error with the Epochs testing dataset (neither epochs nor epochs_full fixtures)

[larsoner]

Okay, I took a quick look -- looks like using raw works

[larsoner]

@drammock looks like the test_ica.py failure seen on the Notebook tests is real, I can replicate locally with pytest mne/viz/tests/test_ica.py:

mne/viz/tests/test_ica.py:222: in test_plot_ica_sources
    fig = ica.plot_sources(raw)
mne/preprocessing/ica.py:1964: in plot_sources
    return plot_ica_sources(self, inst=inst, picks=picks,
mne/viz/ica.py:90: in plot_ica_sources
    fig = _plot_sources(ica, inst, picks, exclude, start=start, stop=stop,
mne/viz/ica.py:1106: in _plot_sources
    fig._draw_traces()
mne/viz/_mpl_figure.py:1721: in _draw_traces
    if self.mne.ch_selections is None and self.mne.butterfly
E   AttributeError: 'BrowserParams' object has no attribute 'ch_selections'

[larsoner]

Thanks @drammock !