Forms :: To Get X-pos Of Item At Radio-change Trigger

i want to get the x-pos of item at the radio-change trigger but it return the error

declare
x_pos number;
lnum_x_pos number;
lnum_width number;
begin

if :radio_group = 'E' then
lnum_x_pos := Get_item_property('Remarks',x_pos);
Message('Position of x...'||lnum_x_pos);
lnum_width := get_item_property('Remarks',width);
Message('width of remarks field..'||lnum_width);

else
set_item_property('remarks',enabled,property_true);
set_item_property('remarks',visible,property_true);

end if;
end;

error is:::

argument to built in get_item_property/field char can not be null?