fix: Avoid NPE in McpClientSession

Author: vorburger

mcp-core/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java

@@ -277,7 +277,15 @@ public <T> Mono<T> sendRequest(String method, Object requestParams, TypeRef<T> t
 					deliveredResponseSink.complete();
 				}
 				else {
-					deliveredResponseSink.next(this.transport.unmarshalFrom(jsonRpcResponse.result(), typeRef));
+					var result = jsonRpcResponse.result();
+					var unmarshalled = this.transport.unmarshalFrom(result, typeRef);
+					// https://github.com/modelcontextprotocol/java-sdk/issues/605
+					if (unmarshalled != null)
+						deliveredResponseSink.next(unmarshalled);
+					else
+						// NB: next() cannot be called with null
+						deliveredResponseSink
+								.error(new IllegalStateException("Failed to unmarshal response result: " + result));
 				}
 			}
 		});