Inappropriate error message displayed when importing an invalid string

[MarinaKisacik]

Found while testing #3541

Steps to Reproduce:

1. Open Macromolecules - Flex mode (clean canvas)
2. Paste the following invalid BILN string from the clipboard: {}
3. Click Open as New or Add to Canvas button

Actual Result:
An inappropriate error message is displayed

Expected Result:
"The string cannot be interpreted as a valid BILN string." error message is displayed

As per Req:

As per BA:

Found in Environment:

• Ketcher Version 3.17.0-rc.1 Build at 2026-06-02; 21:27:10
• Indigo Version 1.45.0-rc.2.0-g168fb85f2-wasm32-wasm-clang-23.0.0
• Chrome Version 149.0.7827.156 (Official Build) (64-bit)
• Win11