[pv generator] missing progress bar

[kswang1029]

Describe the bug
The progress bar with the full screen lock is missing when trying to generate another pv image. When it is missing, we can still interact with the GUI. If we click the PV widget button during the generation, we will see the progress bar and screen lock are back.

Screen.Recording.2024-02-19.at.11.49.40.mov

To Reproduce
Steps to reproduce the behavior:

1. load a large-enough FITS cube so that PV generation process lasts longer for observing the behavior (eg 2k x 2k x 2k)
2. create a pv cut and request a pv image. This should behave normal with a progress bar and screen lock until the generation process is done.
3. reconfigure the pv cut and re-request a pv image generation. We should see the behaviors like the screen recording above.

Expected behavior
The progress bar and screen lock should be displayed while pv generation is ongoing.

Platform info (please complete the following information):

• OS [e.g. macOS Monterey]: macOS Ventura
• Browser [e.g. chrome, safari, electron app]: chrome
• Browser version [e.g. 22]: 121
• Backend branch [e.g. dev, v3b2 release]: dev, v4.1-stable
• Frontend branch [e.g. dev, v3b2 release]: dev, v4.1-stable

Additional context
Add any other context about the problem here.