As discussed on the Isis users list:
Seems reasonable. Would you like to provide a pull request? 
On 2 March 2015 at 16:20, Erik de Hair <email@example.com> wrote:
> Hi Dan,
> On 03/02/2015 03:35 PM, Dan Haywood wrote:
>> Is there any use of setting the selected option to the first item of the
>> choices? It seems a bit strange from the user's perspective.
>> The rationale, IIRC, is that if the current value is not valid in the new
>> set of choices — if(!curr.containedIn(choiceMementos)) — then we set
>> the dependent choice to one of the valid set available as a convenience.
>> I suppose, strictly speaking, that this should only be done if the
>> dependent parameter is mandatory; otherwise a user might want to leave it
>> as null.
> I think, even when the parameter is mandatory, this should be handled by
> validation. It would be even better to set it to null because the user is
> forced to choose an option and would otherwise possibly submit a selected
> value the user didn't want to set (and didn't see it changing).
>> I can reset it in my code but maybe this scenario isn't desirable for any
>>> Isis application.
>>> (Without trying it out for myself), I'm trying to think as to how you'd
>> that ... in the defaultNXxx() method, I am guessing?
>> By copying the ReferencePanel-class to my app Just to try if it
> works when setting it to null.