Summary

MongoAdditionalMappingContributor already checks the two mappings below and would reject both,
but it dereferences a null on the way to the decision. The result is a NullPointerException
naming an internal Hibernate ORM type instead of a FeatureNotSupportedException naming the
unsupported mapping. Both cases are null guards on the same checking path.

Case 1: non-aggregated composite identifier (@IdClass)

A non-aggregated composite identifier leaves PersistentClass.getIdentifierProperty() null, which
MongoAdditionalMappingContributor.checkPropertyTypes passes straight to checkPropertyType.

@Entity @Table(name = "items")
@IdClass(ItemId.class)
static class ItemWithIdClass {
    @Id @ManyToOne @JoinColumn(name = "owner_id") Owner owner;
    @Id int itemId;
}

java.lang.NullPointerException: Cannot invoke "org.hibernate.mapping.Property.getName()" because "property" is null
  at com.mongodb.hibernate.internal.boot.MongoAdditionalMappingContributor.checkPropertyType(MongoAdditionalMappingContributor.java:304)
  at com.mongodb.hibernate.internal.boot.MongoAdditionalMappingContributor.checkPropertyTypes(MongoAdditionalMappingContributor.java:239)

A multi-column primary key is already rejected with FeatureNotSupportedException in
AbstractMqlTranslator.createKeyFilter, so declaring one with @IdClass should be rejected the
same way until HIBERNATE-206 and HIBERNATE-207 land.

Case 2: dynamic model entity

An entity declared without a class is a dynamic model, whose attributes report a null Java type.
MongoAdditionalMappingContributor.forbidUnsupportedTypes passes that to Set.of(...).contains,
which rejects null.

<entity-mappings xmlns="http://www.hibernate.org/xsd/orm/mapping" version="7.0">
    <entity name="Employee" metadata-complete="true">
        <attributes>
            <id name="id"><target>Integer</target></id>
            <basic name="name"><target>String</target></basic>
        </attributes>
    </entity>
</entity-mappings>

java.lang.NullPointerException
  at java.base/java.util.Objects.requireNonNull(Objects.java:209)
  at java.base/java.util.ImmutableCollections$SetN.contains(ImmutableCollections.java:937)
  at com.mongodb.hibernate.internal.boot.MongoAdditionalMappingContributor.forbidUnsupportedTypes(MongoAdditionalMappingContributor.java:332)

Proposed fix

Guard both sites and throw FeatureNotSupportedException naming the unsupported mapping.

Acceptance

Both mappings are rejected with FeatureNotSupportedException whose message names the unsupported
mapping.