Bug report

Bug summary

Typing a dead key (a key which is not written directly after a keypress, e.g. ^, ´, `) into a TextBox widget raises a TypeError. This also occurs when you press CapsLock. This prevents the user from entering some characters like "^" or using CapsLock.

Edit: After about version 3.4 no error is being raised. Now CapsLock works but the TextBox just ignores dead keys.

Code for reproduction

from matplotlib import pyplot as plt
from matplotlib.widgets import TextBox

fig, ax = plt.subplots(figsize=(6, 3))
fig.subplots_adjust(bottom=0.2)
text_box_ax = fig.add_axes([0.4, 0.02, 0.5, 0.06])
text_box = TextBox(text_box_ax, "Type diacritic character (e.g. ^): ")
plt.show()

Actual outcome

Traceback (most recent call last):
  File "\matplotlib\cbook\__init__.py", line 196, in process
    func(*args, **kwargs)
  File "\matplotlib\widgets.py", line 790, in _keypress
    if len(key) == 1:
TypeError: object of type 'NoneType' has no len()

Expected outcome
Normal behaviour of those characters when typing them in editors or searchbars etc.

Matplotlib version

• Operating system: windows 10, build 19041.867
• Matplotlib version: 3.2.2
• Matplotlib backend: TkAgg
• Python version: 3.8
• Jupyter version (if applicable):
• Other libraries: