Copy to clipboard doesn't work if `Multi-Tailed Arrow` present on the canvas

[AlexeyGirin]

Steps to Reproduce

1. Go to Molecules mode
2. Create following configuration on the canvas:
   
3. Select all object on the canvas (press Ctrl+a) and copy it to clipboard (press Ctrl+c)

Actual behavior
System throws exception: Convert error! Given string could not be loaded as (query or plain) molecule or reaction, see the error messages: 'molecule json loader: Not a molecule. Found 1 arrows.', 'array: invalid index 0 (size=0)', 'SEQUENCE loader: Unknown polymer type ''.', 'molecule json loader: Not a molecule. Found 1 arrows.'

Expected behavior
No exception.

Versions

• Ketcher Version 3.3.0-rc.1 Build at 2025-04-17; 05:10:48
• Indigo Version 1.32.0-dev.1.2-gd13f1f79-wasm32-wasm-clang-19.0.0
• Chrome Version 135.0.7049.96 (Official Build) (64-bit)
• Win10

Issue found while testing: #2591