WindowBuilder supports only SmartGWT versions 2.4, 2.5. But 4.1p found

To fix this error substitute file com.google.gdt.eclipse.designer.SmartGWT_3.1.2.r43x201307151456.jar under eclipse/plugin white these (free com.google.gdt.eclipse.designer.SmartGWT_3.1.2.r43x201307151456.jar
Confirm to work with SmartGWT 4.1p + eclipse 4.3(kepler) + gwt-designer 3.1.2. Below modification code details changed on file linked
Daniele

Index: src/com/google/gdt/eclipse/designer/smart/parser/ClassLoaderValidator.java
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src/com/google/gdt/eclipse/designer/smart/parser/ClassLoaderValidator.java (working copy)

 import org.eclipse.jdt.core.IJavaProject;
 
 import org.apache.commons.lang.ArrayUtils;
 import org.apache.commons.lang.StringUtils;
 
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;

 /**
  * {@link IClassLoaderValidator} for SmartGWT.
  * 

  * @coverage SmartGWT
  */
 public final class ClassLoaderValidator implements IClassLoaderValidator {

  private static final Pattern VERSIONS_PATTERN = Pattern.compile("^(\\d+\\.\\d+)[dp]?$");
  private static final String[] VALID_VERSIONS = {"2.4", "2.5", "3.0", "3.1", "4.0", "4.1", "5.0"};
  private static final String VALID_VERSIONS_STRING = StringUtils.join(VALID_VERSIONS, ", ");
 
   ////////////////////////////////////////////////////////////////////////////
   //

       public String runObject() throws Exception {
         Class classVersion = classLoader.loadClass("com.smartgwt.client.Version");
         String version = (String) ReflectionUtils.invokeMethod(classVersion, "getVersion()");
         Matcher matcher = VERSIONS_PATTERN.matcher(version);
         if (matcher.matches() && ArrayUtils.contains(VALID_VERSIONS, matcher.group(1))) {
           initializeSmartGWT(classLoader);
           return null;
         }