ReflectiveMessageValidator reports "Lists must not contain null references." for null values in response to workspace/configuration

[jansorg]

LSP4J 0.23.1

The LSP contains this about the response to the workspace/configuration request:

If the client can’t provide a configuration setting for a given scope then null needs to be present in the returned array.

LSP4J's method signature requires to return a List<Object>, but ReflectiveMessageValidator always seems to report null values in lists as incorrect.

@JsonRequest("workspace/configuration")
	default CompletableFuture<List<Object>> configuration(ConfigurationParams configurationParams) {
		throw new UnsupportedOperationException();
	}

When I return a null configuration value, this is reported by the message validator:

Caused by: org.eclipse.lsp4j.jsonrpc.MessageIssueException: Lists must not contain null references. Path: $.result[0]
	at org.eclipse.lsp4j.jsonrpc.validation.ReflectiveMessageValidator.consume(ReflectiveMessageValidator.java:67)

As far as I understand null values to the configuration request should be accepted by the message validator.

[pisv]

@jansorg Can you try using JsonNull.INSTANCE instead of null?