java.lang.RuntimeException: The context 'Domainmodel returns Domainmodel' is not valid for type 'Domainmodel'
Recommended contexts for type 'Domainmodel': [Domainmodel returns Domainmodel]
The context 'Domainmodel returns Domainmodel' is valid for types: Domainmodel, null

Semantic Object: Domainmodel
URI: platform:/resource/canardnoex.sample/testsimple.canNoEx
at org.eclipse.xtext.serializer.diagnostic.ISerializationDiagnostic$ExceptionThrowingAcceptor.accept(ISerializationDiagnostic.java:131)
at org.xtext.example.canardNoExtends.serializer.CanNoExSemanticSequencer.sequence(CanNoExSemanticSequencer.java:68)
at org.eclipse.xtext.serializer.sequencer.AbstractSemanticSequencer.createSequence(AbstractSemanticSequencer.java:67)
at org.eclipse.xtext.serializer.impl.Serializer.serialize(Serializer.java:118)

...etc

Domainmodel:
    (elements+=Type | links+=Rel)*;

Type: 
Category|Block;

Category:
'cat' nameCat=STRING;

Block: 
'block' name=STRING;

Rel:
'source' src=Block 'destination' des=Block;

package org.xtext.example.mydsl.tests

import com.google.inject.Inject
import com.google.inject.Provider
import org.eclipse.emf.common.util.URI
import org.eclipse.emf.ecore.resource.ResourceSet
import org.eclipse.xtext.serializer.ISerializer
import org.eclipse.xtext.testing.InjectWith
import org.eclipse.xtext.testing.extensions.InjectionExtension
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.^extension.ExtendWith
import org.xtext.example.mydsl.myDsl.MyDslFactory

@ExtendWith(InjectionExtension)
@InjectWith(MyDslInjectorProvider)
class DummyTest {
	
	@Inject extension ISerializer
	@Inject Provider<ResourceSet> rsp
	
	@Test
	def void testIt() {
		val rs = rsp.get
		val r = rs.createResource(URI.createURI("dummy.mydsl"))
		val m = MyDslFactory.eINSTANCE.createDomainmodel=> [
			elements += MyDslFactory.eINSTANCE.createCategory=>[
				nameCat="Hugo"
			]
		]
		r.contents+=m
		println(m.serialize)
	}
	
}