SelectedValueComparator defensively handles null values in exhaustiveCompare

Issue: SPR-12001

spring-webmvc/src/main/java/org/springframework/web/servlet/tags/form/SelectedValueComparator.java

 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2014 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.
@@ -148,7 +148,7 @@ abstract class SelectedValueComparator {
 			PropertyEditor editor, Map<PropertyEditor, Object> convertedValueCache) {
 
 		String candidateDisplayString = ValueFormatter.getDisplayString(candidate, editor, false);
-		if (boundValue.getClass().isEnum()) {
+		if (boundValue != null && boundValue.getClass().isEnum()) {
 			Enum<?> boundEnum = (Enum<?>) boundValue;
 			String enumCodeAsString = ObjectUtils.getDisplayString(boundEnum.name());
 			if (enumCodeAsString.equals(candidateDisplayString)) {