Bruno,
You would only be getting this error if you are attempting to use this Flex Viewer 2.4 widget in an earlier version.
AppEvent.addListener(AppEvent.WIDGET_RUN, checkIfLinkWidget);
ViewerContainer.addEventListener(AppEvent.WIDGET_RUN, checkIfLinkWidget);
Bruno,
Try changing this line:AppEvent.addListener(AppEvent.WIDGET_RUN, checkIfLinkWidget);
To this:ViewerContainer.addEventListener(AppEvent.WIDGET_RUN, checkIfLinkWidget);