Removing a shared axes via ax.remove() leads to an error.

[ImportanceOfBeingErnest]

Bug report

Bug summary

removing a shared axes via ax.remove() leads to an error.

This bisects to "Fix locator/formatter setting when removing shared Axes" #13983 (in current master, targeted for 3.2) which initially headed out to make the situation of removing a shared axes better, fixing #12853, but apparently the test doesn't capture the below simple case.

Hence marking as release critical for 3.2, in order not to release this bug.

Code for reproduction

import matplotlib.pyplot as plt

fig, axs = plt.subplots(2, sharex=True)

axs[0].remove()

plt.show()

Actual outcome

Traceback (most recent call last):
  File "d:\***\matplotlib\lib\matplotlib\backends\backend_qt5.py", line 488, in _draw_idle
    self.draw()
  File "d:\***\matplotlib\lib\matplotlib\backends\backend_agg.py", line 396, in draw
    self.figure.draw(self.renderer)
  File "d:\***\matplotlib\lib\matplotlib\artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "d:\***\matplotlib\lib\matplotlib\figure.py", line 1722, in draw
    renderer, self, artists, self.suppressComposite)
  File "d:\***\matplotlib\lib\matplotlib\image.py", line 136, in _draw_list_compositing_images
    a.draw(renderer)
  File "d:\***\matplotlib\lib\matplotlib\artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "d:\***\matplotlib\lib\matplotlib\axes\_base.py", line 2622, in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
  File "d:\***\matplotlib\lib\matplotlib\image.py", line 136, in _draw_list_compositing_images
    a.draw(renderer)
  File "d:\***\matplotlib\lib\matplotlib\artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "d:\***\matplotlib\lib\matplotlib\axis.py", line 1227, in draw
    ticks_to_draw = self._update_ticks()
  File "d:\***\matplotlib\lib\matplotlib\axis.py", line 1103, in _update_ticks
    major_locs = self.get_majorticklocs()
  File "d:\***\matplotlib\lib\matplotlib\axis.py", line 1348, in get_majorticklocs
    return self.major.locator()
  File "d:\***\matplotlib\lib\matplotlib\ticker.py", line 2032, in __call__
    return self.tick_values(vmin, vmax)
  File "d:\***\matplotlib\lib\matplotlib\ticker.py", line 2040, in tick_values
    locs = self._raw_ticks(vmin, vmax)
  File "d:\***\matplotlib\lib\matplotlib\ticker.py", line 1979, in _raw_ticks
    nbins = np.clip(self.axis.get_tick_space(),
  File "d:\***\matplotlib\lib\matplotlib\axis.py", line 2193, in get_tick_space
    length = ((ends[1][0] - ends[0][0]) / self.axes.figure.dpi) * 72
AttributeError: 'NoneType' object has no attribute 'dpi'

Expected outcome

A plot with the first axes removed, as would be achieved with the same code running in 3.1

Matplotlib version

• Operating system: Win 8.1
• Matplotlib version: current master
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.6