GH-3018: Fix Http NPE when no application context

Resolves #3018

Author: garyrussell

spring-integration-http/src/main/java/org/springframework/integration/http/outbound/AbstractHttpRequestExecutingMessageHandler.java

@@ -34,6 +34,7 @@
 import org.reactivestreams.Publisher;
 
 import org.springframework.beans.factory.BeanFactory;
+import org.springframework.context.ApplicationContext;
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.Expression;
@@ -535,7 +536,9 @@ protected Object evaluateTypeFromExpression(Message<?> requestMessage, @Nullable
 							"evaluation resulted in a " + typeClass + ".");
 			if (type instanceof String && StringUtils.hasText((String) type)) {
 				try {
-					type = ClassUtils.forName((String) type, getApplicationContext().getClassLoader());
+					ApplicationContext applicationContext = getApplicationContext();
+					type = ClassUtils.forName((String) type,
+							applicationContext == null ? null : applicationContext.getClassLoader());
 				}
 				catch (ClassNotFoundException e) {
 					throw new IllegalStateException("Cannot load class for name: " + type, e);