Unable to save RNX featured canvas to SDF2000 format file

**Environment details (please complete the following information):**
- Indigo Toolkit Version 1.18.0-dev.2.0-g369c82f8c-x86_64-linux-gnu-11.4.0
- Ketcher Version 2.17.0-rc.2 Build at 2024-01-03; 17:17:22
- Chrome Version 120.0.6099.130 (Official Build) (64-bit)
- Win10
- Issue found while testing https://github.com/epam/Indigo/issues/1440

**Describe the bug**
Unable to save RNX featured canvas to SDF2000 format file

**Steps to Reproduce**
1. Load from file: [UnableToSaveToSDF2000.zip](https://github.com/epam/Indigo/files/13873622/UnableToSaveToSDF2000.zip) (unzip first)
2. Press Save button and choose MDL SDFfile V2000

**Actual behavior**
System throws exception:
Convert error! IndigoException: core: is not a base molecule
[nightly.test.lifescience.opensource.epam.com-1704802989255.log](https://github.com/epam/Indigo/files/13873669/nightly.test.lifescience.opensource.epam.com-1704802989255.log)

**Expected behavior**
Save works as intended (filter out RNX features and saves the rest)