Describe the bug
If we use the level generator to generate a set of contour levels then render, add a level between these then re-render, then delete the manually added level from the list, we see it is the last level being deleted.

See demo

To Reproduce
Steps to reproduce the behavior:

1. load an image
2. use min-max level generator to get 5 levels and render
3. manually add a level between the first and second levels and render
4. delete that manual level and we will see the it is the last (the 6th) being deleted.

Expected behavior
Correct level being deleted.

Platform info (please complete the following information):

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