Modeling Activity Diagram on Eclipse / News: Recent posts

We are working on the first version of an editor. It is made with the GMF plug-in. We are meeting some problems.
java.lang.NullPointerException
at org.eclipse.emf.codegen.ecore.genmodel.impl.GenTypedElementImpl.getTypeGenPackage(GenTypedElementImpl.java:65)
at org.eclipse.emf.codegen.ecore.genmodel.impl.GenPackageImpl$DependencyHelper.<init>(GenPackageImpl.java:1978)
at org.eclipse.emf.codegen.ecore.genmodel.impl.GenPackageImpl.prepareCache(GenPackageImpl.java:2705)
at org.eclipse.emf.codegen.ecore.genmodel.generator.GenPackageGeneratorAdapter.doPreGenerate(GenPackageGeneratorAdapter.java:136)
at org.eclipse.emf.codegen.ecore.generator.AbstractGeneratorAdapter.preGenerate(AbstractGeneratorAdapter.java:157)
at org.eclipse.emf.codegen.ecore.generator.Generator.generate(Generator.java:327)
at org.eclipse.emf.codegen.ecore.genmodel.presentation.GenModelActionBarContributor$8.execute(GenModelActionBarContributor.java:370)
at org.eclipse.ui.actions.WorkspaceModifyOperation$1.run(WorkspaceModifyOperation.java:101)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:1737)
at org.eclipse.ui.actions.WorkspaceModifyOperation.run(WorkspaceModifyOperation.java:113)
at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:113)... read more