fix(mps-model-adapters): revert "don't reject illegal concept descriptors"

This reverts commit c997cc0.

The name `resolveConcept` suggest that the concept was found and
metamodel information is available. If `MPSLanguageRepository` never
returns null then this conflicts with the metamodel information from
`BuiltinLanguages`.

Author: benedekh

mps-model-adapters/src/main/kotlin/org/modelix/model/mpsadapters/MPSLanguageRepository.kt

@@ -20,6 +20,7 @@ import jetbrains.mps.smodel.adapter.ids.SConceptId
 import jetbrains.mps.smodel.adapter.structure.MetaAdapterFactory
 import jetbrains.mps.smodel.language.ConceptRegistry
 import jetbrains.mps.smodel.language.LanguageRegistry
+import jetbrains.mps.smodel.runtime.illegal.IllegalConceptDescriptor
 import org.jetbrains.mps.openapi.language.SAbstractConcept
 import org.jetbrains.mps.openapi.module.SRepository
 import org.modelix.model.api.IConcept
@@ -38,6 +39,8 @@ data class MPSLanguageRepository(private val repository: SRepository) : ILanguag
 
         val conceptDescriptor = ConceptRegistry.getInstance().getConceptDescriptor(conceptId)
 
+        if (conceptDescriptor is IllegalConceptDescriptor) return null
+
         return MPSConcept(MetaAdapterFactory.getAbstractConcept(conceptDescriptor))
     }