Vaadin update combobox values in a grid

I have a Vaadin (7.6.7) grid, that shows a combobox when in editor mode. Another action can modify the items of the combobox, but the combobox in the grid does not update itself.

grid.getColumn("id").setEditorField(theBox).setConverter(new Converter<String, String>() {
    @Override
    public String convertToModel(String value, Class<? extends String> targetType, Locale locale) throws ConversionException {
        retu value; // not sure for what this is required
    }

    @Override
    public String convertToPresentation(String value, Class<? extends String> targetType, Locale locale) throws ConversionException {
        // don't show the id, but the name
        A a = endpoint.getA(value);
        retu a.getName();
    }

    @Override
    public Class<String> getModelType() {
        retu String.class;
    }

    @Override
    public Class<String> getPresentationType() {
        retu String.class;
    }
});

theBox is a Combobox that is filled before - the initial display is correct (it contains objects of type A)

Now another action manipulates the objects of type A, e.g. delete an object.

How can I update now the combobox in the grid ?

I tried

theBox.removeAllItems();
List<A> as = endpoint.getAs();
teamBox.setContainerDataSource(new BeanItemContainer<>(String.class, as.stream().map(A::getIdent).collect(Collectors.toList())));
for (A each : as) {
    theBox.setItemCaption(a.getId(), a.getName());
}

to call before, but when editing the grid, the theBox shows still the old values.

What do I miss ?