Hi Siva, I had the same problem, maybe this example can help you.
If you use an expression instead of a field when you set the "List Box", the "Always one selected value" property is always disabled.
Excuseme for my bad english.
You can't clear the selected value untill you un-select the option.
Try and Apply > OK like as shown in screen. Hope this will help you.
in list box first select one value
after to go properties ''Always one selected value" ie enable
you selet check box
if you click on clear button that value is never errase
Hi Arif, to do this you just need to make a selection in a list box and then right click and go into properties, then tick 'Always one selected value' in the general tab