[news.eclipse.modeling.gmf] Unhandled event loop exception (problem with

[Jiri Semecky <jiri_semecky@xxxxxxxxxx>]

I'm trying to set the model attribute when an editpart is double-clicked.
The scenario follows:

1. in en EditPart I install an OpenEditPolicy
2. in getOpenCommand execute EMF command and return null for the GEF command

installEditPolicy(EditPolicyRoles.OPEN_ROLE, new OpenEditPolicy() {
  protected Command getOpenCommand(Request request) {
     getEditingDomain().getCommandStack().execute(
        SetCommand.create(
	     getEditingDomain(),
             ((Node)getModel()).getElement(),
             XXXPackage.eINSTANCE.getYYYType_ZZZ(),
             "new_src_value" ) );
     return null;
  }
};

3. I get "Unhandled event loop exception:"
java.lang.IllegalStateException: Cannot activate read/write transaction 
in read-only transaction context

What is the problem?
Could the EMF command (org.eclipse.emf.common.command.Command) be 
somehow encapsulated into a GEF command (org.eclipse.gef.commands.Command)

Thanks for any hint,
--Jirka