Tristate property (true/false/null)

[Steph]

I try to handle a tristate property (true/false/null) with a radiogroup :

       <t:radiogroup id="prop1" value="property1">
           <t:radio value="true"/>true
           <t:radio value="false"/>false
           <t:radio value="null"/>not defined
       </t:radiogroup>

but the selection of "not defined" set my property to true.
Is there a specific way to handle null values ?

Stephane