Google Web Toolkit: Unexpected error while processing XML

I wanna use gwt 2.6.1 , I've had some problems. Does anyone have an idea?

[ERROR] Unexpected error while processing XML

[INFO] java.lang.NoClassDefFoundError: com/google/gwt/core/ext/GeneratorExt

[INFO] at java.lang.ClassLoader.defineClass1(Native Method)

[INFO] at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)

[INFO] at java.lang.ClassLoader.defineClass(ClassLoader.java:615)

[INFO] at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)

[INFO] at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)

[INFO] at java.net.URLClassLoader.access$000(URLClassLoader.java:58)

[INFO] at java.net.URLClassLoader$1.run(URLClassLoader.java:197)

[INFO] at java.security.AccessController.doPrivileged(Native Method)

[INFO] at java.net.URLClassLoader.findClass(URLClassLoader.java:190)

[INFO] at java.lang.ClassLoader.loadClass(ClassLoader.java:306)

[INFO] at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)

[INFO] at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

[INFO] at com.google.gwt.dev.cfg.ModuleDefSchema$ClassAttrCvt.convertToArg(ModuleDefSchema.java:888)

[INFO] at com.google.gwt.dev.util.xml.HandlerArgs.convertToArg(HandlerArgs.java:64)

[INFO] at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:221)

[INFO] at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:294)

[INFO] at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:501)

[INFO] at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:1363)

[INFO] at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2756)

[INFO] at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:647)

[INFO] at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:511)

[INFO] at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:808)

[INFO] at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:737)

[INFO] at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:119)

[INFO] at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1205)

[INFO] at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:522)

[INFO] at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.parse(ReflectiveParser.java:347)

[INFO] at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.access$200(ReflectiveParser.java:68)

[INFO] at com.google.gwt.dev.util.xml.ReflectiveParser.parse(ReflectiveParser.java:418)

[INFO] at com.google.gwt.dev.cfg.ModuleDefLoader.nestedLoad(ModuleDefLoader.java:346)

[INFO] at com.google.gwt.dev.cfg.ModuleDefSchema$BodySchema.__inherits_begin(ModuleDefSchema.java:497)

[INFO] at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)

[INFO] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

[INFO] at java.lang.reflect.Method.invoke(Method.java:597)

[INFO] at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:230)

[INFO] at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:294)

[INFO] at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:501)

[INFO] at com.sun.org.apache.xerces.internal.parsers.AbstractXMLDocumentParser.emptyElement(AbstractXMLDocumentParser.java:179)

[INFO] at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:1343)