[fix][test] Flaky-test: BrokerServiceTest.testShutDownWithMaxConcurrentUnload (#24769)

Author: Shawyeok

pulsar-broker/src/test/java/org/apache/pulsar/broker/service/BrokerServiceTest.java

@@ -179,10 +179,11 @@ public void testShutDownWithMaxConcurrentUnload() throws Exception {
         assertEquals(bundlesData.getNumBundles(), bundleNum);
         List<String> list = admin.brokers().getActiveBrokers("test");
         assertEquals(list.size(), 1);
+        BrokerService brokerService = pulsar.getBrokerService();
         admin.brokers().shutDownBrokerGracefully(1, false);
         //We can only unload one bundle per second, so it takes at least 2 seconds.
         Awaitility.await().atLeast(bundleNum - 1, TimeUnit.SECONDS).untilAsserted(() -> {
-            assertEquals(pulsar.getBrokerService().getTopics().size(), 0);
+            assertEquals(brokerService.getTopics().size(), 0);
         });
         Awaitility.await().atMost(60, TimeUnit.SECONDS).untilAsserted(() -> {
             assertNull(pulsar.getBrokerService());