Bug summary

The figure options modal window causes the figure to freeze.
Similar to #18965.

Code for reproduction

1. Launch jupyter qtconsole
2. execute code

In [1]: %matplotlib qt5

In [2]: from matplotlib.pyplot import plot

In [3]: plot([1, 2, 4])
Out[3]: [<matplotlib.lines.Line2D at 0x22b15172988>]

In [4]: 

3. Click on "Edit axis, curve and image parameters"
4. In qtconsole, execute

import sys

5. Back at the plot properties window, click "OK"
6. Now the figure is frozen and it cannot be closed.

Actual outcome

Figure is frozen.

Expected outcome

Figure is not frozen.

Additional information

I've tested in matplotlib 3.4.3 and 3.5.1

Operating system

macOS 11.6.2

Matplotlib Version

3.5.1

Matplotlib Backend

Qt5Agg

Python version

3.7.6

Jupyter version

IPython          : 7.31.0
ipykernel        : 6.6.1
ipywidgets       : not installed 
jupyter_client   : 7.1.0 
jupyter_core     : 4.9.1 
jupyter_server   : not installed 
jupyterlab       : not installed 
nbclient         : 0.5.9 
nbconvert        : 6.4.0 
nbformat         : 5.1.3 
notebook         : not installed 
qtconsole        : 5.2.2 
traitlets        : 5.1.1

Installation

conda