GH-2425: Fix NPE in ACFactory.shutdownCompleted

Resolves #2425

Assume connection problem when no cause.

**cherry-pick to 2.4.x**
# Conflicts:
#	spring-rabbit/src/test/java/org/springframework/amqp/rabbit/connection/AbstractConnectionFactoryTests.java

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/connection/AbstractConnectionFactory.java

@@ -55,6 +55,7 @@
 import com.rabbitmq.client.Address;
 import com.rabbitmq.client.AddressResolver;
 import com.rabbitmq.client.BlockedListener;
+import com.rabbitmq.client.Method;
 import com.rabbitmq.client.Recoverable;
 import com.rabbitmq.client.RecoveryListener;
 import com.rabbitmq.client.ShutdownListener;
@@ -669,15 +670,18 @@ protected final String getDefaultHostName() {
 
 	@Override
 	public void shutdownCompleted(ShutdownSignalException cause) {
-		int protocolClassId = cause.getReason().protocolClassId();
+		Method reason = cause.getReason();
+		int protocolClassId = RabbitUtils.CONNECTION_PROTOCOL_CLASS_ID_10;
+		if (reason != null) {
+			protocolClassId = reason.protocolClassId();
+		}
 		if (protocolClassId == RabbitUtils.CHANNEL_PROTOCOL_CLASS_ID_20) {
 			this.closeExceptionLogger.log(this.logger, "Shutdown Signal", cause);
 			getChannelListener().onShutDown(cause);
 		}
 		else if (protocolClassId == RabbitUtils.CONNECTION_PROTOCOL_CLASS_ID_10) {
 			getConnectionListener().onShutDown(cause);
 		}
-
 	}
 
 	@Override

spring-rabbit/src/test/java/org/springframework/amqp/rabbit/connection/AbstractConnectionFactoryTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 the original author or authors.
+ * Copyright 2010-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

spring-rabbit/src/test/java/org/springframework/amqp/rabbit/connection/CachingConnectionFactoryTests.java

@@ -1907,4 +1907,39 @@ void testResolver() throws Exception {
 		verify(mockConnectionFactory).newConnection(any(ExecutorService.class), eq(resolver), anyString());
 	}
 
+	@Test
+	void nullShutdownCause() {
+		com.rabbitmq.client.ConnectionFactory mockConnectionFactory = mock(com.rabbitmq.client.ConnectionFactory.class);
+		AbstractConnectionFactory cf = createConnectionFactory(mockConnectionFactory);
+		AtomicBoolean connShutDown = new AtomicBoolean();
+		cf.addConnectionListener(new ConnectionListener() {
+
+			@Override
+			public void onCreate(Connection connection) {
+			}
+
+			@Override
+			public void onShutDown(ShutdownSignalException signal) {
+				connShutDown.set(true);
+			}
+
+		});
+		AtomicBoolean chanShutDown = new AtomicBoolean();
+		cf.addChannelListener(new ChannelListener() {
+
+			@Override
+			public void onCreate(Channel channel, boolean transactional) {
+			}
+
+			@Override
+			public void onShutDown(ShutdownSignalException signal) {
+				chanShutDown.set(true);
+			}
+
+		});
+		cf.shutdownCompleted(new ShutdownSignalException(false, false, null, chanShutDown));
+		assertThat(connShutDown.get()).isTrue();
+		assertThat(chanShutDown.get()).isFalse();
+	}
+
 }