Fail cleanly when submitted node data misses the discriminator field #32
Description
Problem
When submitted node data does not contain the discriminator field, the component currently reaches undefined array access and then fails with low-level errors.
Current behavior
A malformed or incomplete submitted node can trigger PHP warnings or generic runtime failures instead of one clear form-related exception.
Expected behavior
Submitted node data should be validated early and fail with a clean, domain-specific exception that explains that the discriminator field is required.
Why it matters
This component is used in dynamic editors and CMS flows, where partial or malformed frontend payloads are realistic and should fail in a predictable way.