Export of ambiguous monomers to Sequence/FASTA doesn't work for mixed ambiguous monomers

[AlexeyGirin]

Steps to Reproduce

1. Toggle to Macro - Flex
2. Load following HELM using copy from clipboard way: PEPTIDE1{(A+C+D+E+F+G+H+I+K+L+M+N+O+P+Q+R+S+T+U+V+W+Y)}$$$$V2.0
   
3. Try to export canvas to Sequence (or FASTA - result is the same)

Actual behavior
System throws exception: Convert error! map::at: key not found

Expected behavior
No exception. Correct error message should be shown

Versions

• Ketcher Version 2.25.0-rc.5 Build at 2024-09-24; 18:17:25
• Indigo Toolkit Version 1.24.0-rc.4.0-gdf51dc428-wasm32-wasm-clang-19.0.0
• Chrome Version 129.0.6668.71 (Official Build) (64-bit)
• Win10

Reference issues: #2015