GH-3319: Fix NPE in the KafkaListenerEndpointRegistry (#3320)

The `KafkaListenerEndpointRegistry.getUnregisteredListenerContainer()`
returns `null` when container is already present in the internal `unregisteredContainers` cache

* Fix `KafkaListenerEndpointRegistry.getUnregisteredListenerContainer()` to return a container
instance from the `unregisteredContainers` cache

(cherry picked from commit 8096c9d)

Author: artembilan

spring-kafka/src/main/java/org/springframework/kafka/config/KafkaListenerEndpointRegistry.java

@@ -168,7 +168,7 @@ public MessageListenerContainer getUnregisteredListenerContainer(String id) {
 			refreshContextContainers();
 			return this.unregisteredContainers.get(id);
 		}
-		return null;
+		return container;
 	}
 
 	/**

spring-kafka/src/test/java/org/springframework/kafka/config/KafkaListenerEndpointRegistryTests.java

@@ -35,11 +35,15 @@
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
 
+import org.springframework.context.support.GenericApplicationContext;
+import org.springframework.kafka.listener.ConcurrentMessageListenerContainer;
 import org.springframework.kafka.listener.MessageListenerContainer;
 
 /**
  * @author Gary Russell
  * @author Joo Hyuk Kim
+ * @author Artem Bilan
+ *
  * @since 2.8.9
  */
 public class KafkaListenerEndpointRegistryTests {
@@ -139,6 +143,21 @@ void getListenerContainersMatchingBiPredicate(List<String> names, BiPredicate<St
 		assertThat(listeners).hasSize(expectedCount);
 	}
 
+	@Test
+	void verifyUnregisteredListenerContainer() {
+		KafkaListenerEndpointRegistry registry = new KafkaListenerEndpointRegistry();
+		GenericApplicationContext applicationContext = new GenericApplicationContext();
+		ConcurrentMessageListenerContainer<?, ?> listenerContainerMock = mock(ConcurrentMessageListenerContainer.class);
+		given(listenerContainerMock.getListenerId()).willReturn("testListenerContainer");
+		applicationContext.registerBean(ConcurrentMessageListenerContainer.class, () -> listenerContainerMock);
+		applicationContext.refresh();
+		registry.setApplicationContext(applicationContext);
+		// Lazy-load from application context
+		assertThat(registry.getUnregisteredListenerContainer("testListenerContainer")).isNotNull();
+		// From internal map
+		assertThat(registry.getUnregisteredListenerContainer("testListenerContainer")).isNotNull();
+	}
+
 	/**
 	 * Provides parameters for the getListenerContainersMatchingBiPredicate test.
 	 * Each set of parameters includes a list of names, a bi-predicate, and the expected count of matching containers.