Fix ErrorMessageExceptionTypeRouter for new Nullability

* Fix `KotlinIntegrationFlowDefinition` for new `RouterSpec` expectations

**Auto-cherry-pick to `7.0.x`**

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/router/AbstractMappingMessageRouter.java

@@ -369,7 +369,9 @@ private void addChannel(Collection<MessageChannel> channels, Message<?> message,
 
 	private void addToCollection(Collection<MessageChannel> channels, Collection<?> channelKeys, Message<?> message) {
 		for (Object channelKey : channelKeys) {
-			addChannelKeyToCollection(channels, message, channelKey);
+			if (channelKey != null) {
+				addChannelKeyToCollection(channels, message, channelKey);
+			}
 		}
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/router/ErrorMessageExceptionTypeRouter.java

@@ -23,6 +23,8 @@
 import java.util.Properties;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.jmx.export.annotation.ManagedAttribute;
 import org.springframework.jmx.export.annotation.ManagedOperation;
 import org.springframework.messaging.Message;
@@ -69,7 +71,6 @@ private void populateClassNameMapping(Set<String> classNames) {
 
 	private Class<?> resolveClassFromName(String className) {
 		try {
-			Assert.state(getApplicationContext() != null, "An ApplicationContext is required");
 			return ClassUtils.forName(className, getApplicationContext().getClassLoader());
 		}
 		catch (ClassNotFoundException e) {
@@ -112,7 +113,7 @@ protected void onInit() {
 	}
 
 	@Override
-	protected List<Object> getChannelKeys(Message<?> message) {
+	protected @Nullable List<Object> getChannelKeys(Message<?> message) {
 		String mostSpecificCause = null;
 		Object payload = message.getPayload();
 		if (payload instanceof Throwable cause) {
@@ -127,7 +128,7 @@ protected List<Object> getChannelKeys(Message<?> message) {
 				cause = cause.getCause();
 			}
 		}
-		return Collections.singletonList(mostSpecificCause);
+		return mostSpecificCause == null ? null : Collections.singletonList(mostSpecificCause);
 	}
 
 }

spring-integration-core/src/main/kotlin/org/springframework/integration/dsl/KotlinIntegrationFlowDefinition.kt

@@ -724,7 +724,7 @@ class KotlinIntegrationFlowDefinition(@PublishedApi internal val delegate: Integ
 	 */
 	fun route(
 		beanName: String, method: String?,
-		routerConfigurer: KotlinRouterSpec<Any?, MethodInvokingRouter>.() -> Unit
+		routerConfigurer: KotlinRouterSpec<Any, MethodInvokingRouter>.() -> Unit
 	) {
 
 		this.delegate.route(beanName, method) { routerConfigurer(KotlinRouterSpec(it)) }
@@ -744,7 +744,7 @@ class KotlinIntegrationFlowDefinition(@PublishedApi internal val delegate: Integ
 	 */
 	fun route(
 		service: Any, methodName: String?,
-		routerConfigurer: KotlinRouterSpec<Any?, MethodInvokingRouter>.() -> Unit
+		routerConfigurer: KotlinRouterSpec<Any, MethodInvokingRouter>.() -> Unit
 	) {
 
 		this.delegate.route(service, methodName) { routerConfigurer(KotlinRouterSpec(it)) }
@@ -754,9 +754,9 @@ class KotlinIntegrationFlowDefinition(@PublishedApi internal val delegate: Integ
 	 * Populate the [ExpressionEvaluatingRouter] for provided SpEL expression
 	 * with provided options from [KotlinRouterSpec].
 	 */
-	fun <T : Any?> route(
+	fun route(
 		expression: String,
-		routerConfigurer: KotlinRouterSpec<T, ExpressionEvaluatingRouter>.() -> Unit = {}
+		routerConfigurer: KotlinRouterSpec<Any, ExpressionEvaluatingRouter>.() -> Unit = {}
 	) {
 
 		this.delegate.route(expression) { routerConfigurer(KotlinRouterSpec(it)) }
@@ -768,7 +768,7 @@ class KotlinIntegrationFlowDefinition(@PublishedApi internal val delegate: Integ
 	 */
 	fun route(
 		messageProcessorSpec: MessageProcessorSpec<*>,
-		routerConfigurer: KotlinRouterSpec<Any?, MethodInvokingRouter>.() -> Unit = {}
+		routerConfigurer: KotlinRouterSpec<Any, MethodInvokingRouter>.() -> Unit = {}
 	) {
 
 		this.delegate.route(messageProcessorSpec) { routerConfigurer(KotlinRouterSpec(it)) }