Impossible to import a valid BILN string - error occurs

Found while testing #3541

**Steps to Reproduce:**

1. Open Macromolecules - Flex mode (clean canvas)
2. Paste the following BILN string from the clipboard: `D(1,2)-C-A-E(1,1)`
3. Click `Open as New` or `Add to Canvas` button

**Actual Result:**
Error appears

**Expected Result:**
String is loaded successfully

As per reqs:

**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 149.0.7827.103 (Official Build) (64-bit)
- Win11

Reproduce the failure in Macromolecules Flex mode using the BILN string `D(1,2)-C-A-E(1,1)` and both `Open as New` and `Add to Canvas`. Trace the import path behind those buttons and compare the parser result with the expected successful load; done means the valid string loads without an error.

Written by the indexing model from the issue text.