Serializing model resource fails with VS Code client

What I try to do:
I want to use the Xtext generator to have a serialized version of my AST next to the actual file, so I have implemented it like this:

override void doGenerate(Resource resource, IFileSystemAccess2 fsa, IGeneratorContext context) {
	val rset = new ResourceSetImpl
	val xmlResource = rset.createResource(resource.URI.appendFileExtension("xml"))
	xmlResource.contents.add(resource.contents.head)
	xmlResource.save(Collections.EMPTY_MAP)
}

When I open a file in VS Code now, I get these exceptions:

SEVERE: Internal error: java.lang.NullPointerException
java.util.concurrent.CompletionException: java.lang.NullPointerException
	at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:331)
	at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:346)
	at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:632)
	at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:506)
	at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2088)
	at org.eclipse.xtext.ide.server.concurrent.WriteRequest.run(WriteRequest.java:44)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.NullPointerException
	at org.my.dsl.generator.SglGenerator.doGenerate(SglGenerator.java:28)
	at org.eclipse.xtext.generator.GeneratorDelegate.doGenerate(GeneratorDelegate.java:43)
	at org.eclipse.xtext.generator.GeneratorDelegate.generate(GeneratorDelegate.java:34)
	at org.eclipse.xtext.build.IncrementalBuilder$InternalStatefulIncrementalBuilder.generate(IncrementalBuilder.java:340)
	at org.eclipse.xtext.build.IncrementalBuilder$InternalStatefulIncrementalBuilder.lambda$launch$2(IncrementalBuilder.java:277)
	at com.google.common.collect.Iterators$6.transform(Iterators.java:783)
	at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:47)
	at com.google.common.collect.FluentIterable.copyInto(FluentIterable.java:791)
	at org.eclipse.xtext.build.ClusteringStorageAwareResourceLoader.executeClustered(ClusteringStorageAwareResourceLoader.java:68)
	at org.eclipse.xtext.build.BuildContext.executeClustered(BuildContext.java:54)
	at org.eclipse.xtext.build.IncrementalBuilder$InternalStatefulIncrementalBuilder.launch(IncrementalBuilder.java:258)
	at org.eclipse.xtext.build.IncrementalBuilder.build(IncrementalBuilder.java:403)
	at org.eclipse.xtext.build.IncrementalBuilder.build(IncrementalBuilder.java:385)
	at org.eclipse.xtext.ide.server.ProjectManager.doBuild(ProjectManager.java:105)
	at org.eclipse.xtext.ide.server.ProjectManager.doInitialBuild(ProjectManager.java:96)
	at org.eclipse.xtext.ide.server.BuildManager.doInitialBuild(BuildManager.java:172)
	at org.eclipse.xtext.ide.server.WorkspaceManager.refreshWorkspaceConfig(WorkspaceManager.java:172)
	at org.eclipse.xtext.ide.server.WorkspaceManager.initialize(WorkspaceManager.java:145)
	at org.eclipse.xtext.ide.server.LanguageServerImpl.lambda$initialize$0(LanguageServerImpl.java:220)
	at org.eclipse.xtext.ide.server.concurrent.WriteRequest.run(WriteRequest.java:38)
	... 5 more

[Error - 14:16:20] Server initialization failed.
  Message: Internal error.
  Code: -32603 
java.util.concurrent.CompletionException: java.lang.NullPointerException
	at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:331)
	at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:346)
	at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:632)
	at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:506)
	at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2088)
	at org.eclipse.xtext.ide.server.concurrent.WriteRequest.run(WriteRequest.java:44)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.NullPointerException
	at org.my.dsl.sgl.generator.SglGenerator.doGenerate(SglGenerator.java:28)
	at org.eclipse.xtext.generator.GeneratorDelegate.doGenerate(GeneratorDelegate.java:43)
	at org.eclipse.xtext.generator.GeneratorDelegate.generate(GeneratorDelegate.java:34)
	at org.eclipse.xtext.build.IncrementalBuilder$InternalStatefulIncrementalBuilder.generate(IncrementalBuilder.java:340)
	at org.eclipse.xtext.build.IncrementalBuilder$InternalStatefulIncrementalBuilder.lambda$launch$2(IncrementalBuilder.java:277)
	at com.google.common.collect.Iterators$6.transform(Iterators.java:783)
	at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:47)
	at com.google.common.collect.FluentIterable.copyInto(FluentIterable.java:791)
	at org.eclipse.xtext.build.ClusteringStorageAwareResourceLoader.executeClustered(ClusteringStorageAwareResourceLoader.java:68)
	at org.eclipse.xtext.build.BuildContext.executeClustered(BuildContext.java:54)
	at org.eclipse.xtext.build.IncrementalBuilder$InternalStatefulIncrementalBuilder.launch(IncrementalBuilder.java:258)
	at org.eclipse.xtext.build.IncrementalBuilder.build(IncrementalBuilder.java:403)
	at org.eclipse.xtext.build.IncrementalBuilder.build(IncrementalBuilder.java:385)
	at org.eclipse.xtext.ide.server.ProjectManager.doBuild(ProjectManager.java:105)
	at org.eclipse.xtext.ide.server.ProjectManager.doInitialBuild(ProjectManager.java:96)
	at org.eclipse.xtext.ide.server.BuildManager.doInitialBuild(BuildManager.java:172)
	at org.eclipse.xtext.ide.server.WorkspaceManager.refreshWorkspaceConfig(WorkspaceManager.java:172)
	at org.eclipse.xtext.ide.server.WorkspaceManager.initialize(WorkspaceManager.java:145)
	at org.eclipse.xtext.ide.server.LanguageServerImpl.lambda$initialize$0(LanguageServerImpl.java:220)
	at org.eclipse.xtext.ide.server.concurrent.WriteRequest.run(WriteRequest.java:38)
	... 5 more

Any idea what I do wrong?

Start at org.my.dsl.generator.SglGenerator.doGenerate, especially line 28, and compare the ResourceSetImpl/createResource/save sequence with the VS Code initialization stack trace. Reproduce the failure when opening a file and trace which value is null; done means model serialization completes without the NullPointerException and the language server initializes successfully.