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 version3.4 no error is being raised. Now CapsLock works but the TextBox just ignores dead keys.

Code for reproduction

frommatplotlibimportpyplotaspltfrommatplotlib.widgetsimportTextBoxfig,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: