Protect sendHeartbeat from cancellation

Issue: SPR-14564

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/session/AbstractSockJsSession.java

@@ -252,6 +252,10 @@ public abstract class AbstractSockJsSession implements SockJsSession {
 		if (isActive()) {
 			if (heartbeatLock.tryLock()) {
 				try {
+					if (this.heartbeatTask == null) {
+						// Cancelled while waiting to acquire the lock
+						return;
+					}
 					writeFrame(SockJsFrame.heartbeatFrame());
 					scheduleHeartbeat();
 				}