Error when closing first of several pyplot figures in TkAgg

[ImportanceOfBeingErnest]

Bug report

Bug summary

Closing the first of two pyplot figures in TkAgg backend, throws an error.

Code for reproduction

import matplotlib
matplotlib.use("TkAgg")
import matplotlib.pyplot as plt

fig1 = plt.figure()
plt.plot([1,2], 'o')

fig2 = plt.figure()
plt.plot([2,1], 'o')

plt.close(fig1)

plt.show()

Actual outcome

in py2 (2.2.3):

Traceback (most recent call last):
  File "D:\xxx\test\closefigureserror.py", line 19, in <module>
    plt.show()
  File "c:\***\lib\site-packages\matplotlib\pyplot.py", line 253, in show
    return _show(*args, **kw)
  File "c:\***\lib\site-packages\matplotlib\backend_bases.py", line 208, in show
    cls.mainloop()
  File "c:\***\lib\site-packages\matplotlib\backends\_backend_tk.py", line 1073, in mainloop
    Tk.mainloop()
  File "c:\***\lib\lib-tk\Tkinter.py", line 414, in mainloop
    _default_root.tk.mainloop(n)
AttributeError: 'NoneType' object has no attribute 'tk'

in py3 (master):

Traceback (most recent call last):
  File "D:\xxx\test\closefigureserror.py", line 19, in <module>
    plt.show()
  File "d:\***\matplotlib\lib\matplotlib\pyplot.py", line 254, in show
    return _show(*args, **kw)
  File "d:\***\matplotlib\lib\matplotlib\backend_bases.py", line 3275, in show
    cls.mainloop()
  File "d:\***\matplotlib\lib\matplotlib\backends\_backend_tk.py", line 1040, in mainloop
    Tk.mainloop()
  File "D:\***\Miniconda3\envs\mplgit\lib\tkinter\__init__.py", line 557, in mainloop
    _default_root.tk.mainloop(n)
AttributeError: 'NoneType' object has no attribute 'tk'

The code works fine in Qt4Agg (with py2) or Qt5Agg (with py3)

Expected outcome

No error.

Matplotlib version

• Operating system: Win 8
• Matplotlib version: 2.2.3 / master
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg
• Python version: 2.7 / 3.6
• Tcl/Tk version: 8.6

[anntzer]

This can be repro'd without mpl:

import tkinter as tk
t1 = tk.Tk()
t2 = tk.Tk()
t1.destroy()
tk.mainloop()

I guess the correct fix is to call mainloop() on the first Tk() that's still alive...

[anntzer]

Looks like

diff --git i/lib/matplotlib/backends/_backend_tk.py w/lib/matplotlib/backends/_backend_tk.py
index a11e65409..c88ad48b4 100644
--- i/lib/matplotlib/backends/_backend_tk.py
+++ w/lib/matplotlib/backends/_backend_tk.py
@@ -1037,4 +1037,6 @@ class _BackendTk(_Backend):
 
     @staticmethod
     def mainloop():
-        tk.mainloop()
+        managers = Gcf.get_all_fig_managers()
+        if managers:
+            managers[0].window.mainloop()

fixes the problem.
If that works for you, can to make a PR out of it?