java.lang.IllegalArgumentException: Tried to pass null folder arg
        at org.openide.filesystems.FileUtil.isParentOf(FileUtil.java:1204)
        at org.netbeans.modules.web.freeform.WebModules$FFWebModule.contains(WebModules.java:322)
        at org.netbeans.modules.web.freeform.WebModules$2.run(WebModules.java:144)
        at org.netbeans.modules.web.freeform.WebModules$2.run(WebModules.java:140)
        at org.openide.util.Mutex.readAccess(Mutex.java:327)
        at org.openide.util.Mutex$1R.run(Mutex.java:1299)
        at org.netbeans.api.project.ProjectManager$2$1.run(ProjectManager.java:120)
        at org.openide.filesystems.EventControl.runAtomicAction(EventControl.java:120)
        at org.openide.filesystems.FileSystem.runAtomicAction(FileSystem.java:499)
        at org.openide.filesystems.FileUtil.runAtomicAction(FileUtil.java:215)
        at org.netbeans.api.project.ProjectManager$2.execute(ProjectManager.java:118)
        at org.openide.util.Mutex.doWrapperAccess(Mutex.java:1320)
        at org.openide.util.Mutex.readAccess(Mutex.java:275)
        at org.netbeans.modules.web.freeform.WebModules.findClassPath(WebModules.java:139)
        at org.netbeans.modules.java.freeform.LookupMergerImpl$ClassPathProviderImpl.findClassPath(LookupMergerImpl.java:77)
        at org.netbeans.modules.java.project.ProjectClassPathProvider.findClassPath(ProjectClassPathProvider.java:64)
        at org.netbeans.api.java.classpath.ClassPath.getClassPath(ClassPath.java:538)
        at org.netbeans.modules.java.source.tasklist.TaskCache.isInError(TaskCache.java:330)
        at org.netbeans.modules.java.source.tasklist.ErrorAnnotator$1.run(ErrorAnnotator.java:309)
        at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:572)
        at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:997)