shutdown thread pool

spring-cloud-starter-tencent-polaris-discovery/src/main/java/com/tencent/cloud/polaris/registry/PolarisServiceRegistry.java

@@ -220,6 +220,8 @@ public class PolarisServiceRegistry implements ServiceRegistry<Registration>, Di
 
 	@Override
 	public void destroy() throws Exception {
+		if (heartbeatExecutor != null) {
 			heartbeatExecutor.shutdown();
 		}
 	}
+}