[BUG] Plotting export values window remains open after closing MDANSE

[ChiCheng45]

Background information
Your OS: Windows 11
Python version: 3.11
MDANSE version: 2.0.1.post13+git.0c6b336
MDANSE_GUI version: 2.0.1.post13+git.0c6b336a
CPU architecture: AMD64

What were you doing?
Plotting

What went wrong?
Plotting export value window remains open after closing MDANSE.

Additional details (optional)