URGENT: Fix startup issue for language server caused by lsp4j 0.15 upgrade

[themkat]

Sorry @fwcd , seems like I was too quick to approve a recent PR:
#381

lsp4j 0.15 causes the following startup issue in many clients:

Exception in thread "main" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
	at org.eclipse.lsp4j.jsonrpc.services.GenericEndpoint.lambda$recursiveFindRpcMethods$2(GenericEndpoint.java:83)
	at org.eclipse.lsp4j.jsonrpc.services.AnnotationUtil.findDelegateSegments(AnnotationUtil.java:32)
	at org.eclipse.lsp4j.jsonrpc.services.AnnotationUtil.findDelegateSegments(AnnotationUtil.java:28)
	at org.eclipse.lsp4j.jsonrpc.services.GenericEndpoint.recursiveFindRpcMethods(GenericEndpoint.java:74)
	at org.eclipse.lsp4j.jsonrpc.services.GenericEndpoint.<init>(GenericEndpoint.java:54)
	at org.eclipse.lsp4j.jsonrpc.services.ServiceEndpoints.toEndpoint(ServiceEndpoints.java:72)
	at org.eclipse.lsp4j.jsonrpc.Launcher$Builder.createRemoteEndpoint(Launcher.java:348)
	at org.eclipse.lsp4j.jsonrpc.Launcher$Builder.create(Launcher.java:320)
	at org.eclipse.lsp4j.launch.LSPLauncher.createServerLauncher(LSPLauncher.java:90)
	at org.javacs.kt.MainKt.main(Main.kt:44)
Caused by: java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:119)
	at java.base/java.lang.reflect.Method.invoke(Method.java:577)
	at org.eclipse.lsp4j.jsonrpc.services.GenericEndpoint.lambda$recursiveFindRpcMethods$2(GenericEndpoint.java:76)
	... 9 more
Caused by: java.lang.UnsupportedOperationException
	at org.eclipse.lsp4j.services.LanguageServer.getNotebookDocumentService(LanguageServer.java:93)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
	... 11 more

The issue has been fixed in a later commit in lsp4j (probably included in next version 0.16). I was able to do the same in our language server and fix the startup issues.
eclipse-lsp4j/lsp4j@04b0c61

Emacs gives a warning of a null value, but ignores it. VSCode works like a charm when I tested it just now.

Again, sorry for not noticing this before I approved the other PR 🙁

[fwcd]

Thanks a lot for finding and fixing this! Curious that this didn't come up during the test suite.