NPE when hovering over a spelling info marker in vs-code

[dhuebner]

Hovering over a spelling info marker or calling Quick Fix results in NPE. Need to safely handle this case:

Caused by: java.lang.NullPointerException: Cannot invoke "org.eclipse.lsp4j.jsonrpc.messages.Either.getLeft()" because the return value of "org.eclipse.lsp4j.Diagnostic.getCode()" is null
	at io.typefox.yang.ide.codeAction.CodeActionService.getCodeActions(CodeActionService.java:33)
	at org.eclipse.xtext.ide.server.LanguageServerImpl.lambda$codeAction$38(LanguageServerImpl.java:790)
	at org.eclipse.xtext.ide.server.WorkspaceManager.doRead(WorkspaceManager.java:438)
	at org.eclipse.xtext.ide.server.LanguageServerImpl.codeAction(LanguageServerImpl.java:781)
	at org.eclipse.xtext.ide.server.LanguageServerImpl.lambda$codeAction$37(LanguageServerImpl.java:767)
	at org.eclipse.xtext.ide.server.concurrent.ReadRequest.lambda$doRun$0(ReadRequest.java:66)
	... 5 more