java.lang.NullPointerException
	at org.eclipse.xtext.ide.serializer.impl.PartialSerializer$SerializeRecursiveStrategy.serialize(PartialSerializer.java:155)
	at org.eclipse.xtext.ide.serializer.impl.PartialSerializer.serializeChanges(PartialSerializer.java:280)
	at org.eclipse.xtext.ide.serializer.impl.RecordingXtextResourceUpdater.applyChange(RecordingXtextResourceUpdater.java:80)
	at org.eclipse.xtext.ide.serializer.impl.ChangeSerializer.endRecordChanges(ChangeSerializer.java:162)
	at org.eclipse.xtext.ide.serializer.impl.ChangeSerializer.applyModifications(ChangeSerializer.java:93)
	at org.eclipse.xtext.ui.editor.model.edit.BatchModification.applyInWorkspace(BatchModification.java:142)

grammar org.xtext.example.mydsl.MyDsl with org.eclipse.xtext.common.Terminals

generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"

Model:
	classes+=Class* statements+=Statement+;
	
Class:
	'class' name=ID '{' vars+=Variable+ '}';
	
Variable: name=ID;
	
Statement: var=[Variable|VariableQN] '=' INT;	

VariableQN hidden(): ID '.' ID;

	@Fix(Diagnostic.LINKING_DIAGNOSTIC)
	def void fixMissingEntity(Issue issue, IssueResolutionAcceptor acceptor) {
		if (issue.message.contains("Variable")) {
			fixMissingVariable(issue, acceptor);
		}
	}

	private def fixMissingVariable(Issue issue, IssueResolutionAcceptor acceptor) {
		acceptor.acceptMulti(
			issue,
			"Create missing variable definition",
			"Create missing variable definition",
			"Entity.gif",
			[ Statement element, ICompositeModificationContext<Statement> context |
				val fullReferenceName = getIssueMissingReferenceName(context.issue)
				val nameArray = fullReferenceName.split("\\.")
				val className = nameArray.get(0)
				val fieldName = nameArray.get(1)
				
				val root = element.eContainer as Model
				
				context.addModification(root, [ctx |
					var cl = root.classes.findFirst[it.name == className]
					
					if (cl === null) {
						// The class didn't exist
						cl =  MyDslFactory::eINSTANCE.createClass() => [
							name = className
						];
						
						// before adding the class, we can add the field
						addVariable(cl, fieldName)
						
						ctx.classes.add(cl)

						// after adding the class, adding the field will cause NullPointerException in Serialization
						// addVariable(cl, fieldName)
					} else {
						// The class existed (maybe created in a previous multiquickfix)
						// NullPointerException if 2 fixs are applied together and the parent class was added in the first fix
						addVariable(cl, fieldName)
					}
				])
				
			]
		)
	}
	
	protected def boolean addVariable(Class cl, String fieldName) {
		val atr = MyDslFactory::eINSTANCE.createVariable() => [
			name = fieldName
		];
		cl.vars.add(atr)
	}
	
	private def String getIssueMissingReferenceName(Issue issue) {
		// Not the best way ...
		
		val issueMessage = issue.message
		val j = issueMessage.lastIndexOf("'")
		val i = issueMessage.lastIndexOf("'", j - 1)
		
		issueMessage.substring(i+1, j)
	}

// aplying the fix to this 2 problems together works because they create their own classes
Class1.test1 = 1 
Class2.test2 = 1

// aplying the fix to this 2 problems together don't work because the first adds the class plus field and the second adds another field to the new class
Class3.test3 = 1 
Class3.test4 = 1