Open Security app.
Click "Groups" tab.
Click "Edit group".
> Nothing happens in ui.
> get a stack trace.

2013-01-07 13:50:38,865 WARN info.magnolia.ui.framework.event.SimpleEventBus : Exception caught when dispatching event: null
com.vaadin.data.Buffered$SourceException
at com.vaadin.ui.AbstractField.setPropertyDataSource(AbstractField.java:634)
at info.magnolia.ui.admincentral.field.builder.AbstractFieldBuilder.setPropertyDataSource(AbstractFieldBuilder.java:126)
at info.magnolia.ui.admincentral.field.builder.SelectFieldBuilder.setPropertyDataSource(SelectFieldBuilder.java:204)
at info.magnolia.ui.admincentral.field.builder.AbstractFieldBuilder.getField(AbstractFieldBuilder.java:100)
at info.magnolia.ui.admincentral.form.builder.FormBuilder.buildForm(FormBuilder.java:87)
at info.magnolia.ui.admincentral.form.FormPresenterImpl.start(FormPresenterImpl.java:125)
at info.magnolia.ui.admincentral.dialog.FormDialogPresenterImpl.start(FormDialogPresenterImpl.java:104)
at info.magnolia.ui.admincentral.dialog.action.EditDialogAction.execute(EditDialogAction.java:85)
at info.magnolia.ui.admincentral.actionbar.ActionbarPresenter.createAndExecuteAction(ActionbarPresenter.java:286)
at info.magnolia.ui.admincentral.workbench.ContentWorkbenchPresenter$2.onActionbarItemClicked(ContentWorkbenchPresenter.java:151)
at info.magnolia.ui.admincentral.event.ActionbarItemClickedEvent.dispatch(ActionbarItemClickedEvent.java:65)
at info.magnolia.ui.admincentral.event.ActionbarItemClickedEvent.dispatch(ActionbarItemClickedEvent.java:43)
at info.magnolia.ui.framework.event.SimpleEventBus.fireEvent(SimpleEventBus.java:78)

Caused by:

Caused by: com.vaadin.data.util.converter.Converter$ConversionException: Unable to convert value of type java.util.ArrayList to presentation type interface java.util.Set. No converter is set and the types are not compatible.
at com.vaadin.data.util.converter.ConverterUtil.convertFromModel(ConverterUtil.java:98)
at com.vaadin.ui.AbstractField.convertFromModel(AbstractField.java:705)
at com.vaadin.ui.AbstractField.convertFromModel(AbstractField.java:690)
at com.vaadin.ui.AbstractField.setPropertyDataSource(AbstractField.java:626)