Crash on undoing after converting and editing parameter

[rodolforg]

Describe the bug Synfig Studio crashes when undoing a layer creation after converting and changing one of its parameter.

To Reproduce Steps to reproduce the behavior:

1. Create a Circle layer (circle, region, outline, it does not matter)
2. Convert its Origin to Radial Composite (or Composite or Scale, ...)
3. In the Parameter Panel, change a converter parameter. If Radial Composite, its angle to 10, for example
4. Change that converter parameter again. If Radial Composite, its angle to 30, for example
5. Change that converter parameter again. If Radial Composite, its angle to 50, for example
6. Undo (50 -> 30)
7. Undo (30 -> 10)
8. Undo (10 -> original value)
9. Undo (converted -> unconverted)
10. Undo (circle layer creation -> none)
11. Crash

System information:

• OS: Linux - Debian
• OS version: testing (Trixie)
• Synfig version: >= 1.3.9 for sure. Version 1.2.2 works fine. I couldn't test 1.3.x for x < 9.