Sorry, explanation required: The code compares the "value" attribute of the widget with the input value. As the input value is None, and you have not specified the attribute value, this is also None. Therefore the "checked" attribute is inserted. The Bool validator returns True or False, neither of which equal None and therefore the attribute "checked" is missing. Therefore always set the "value" attribute to an explicit value.
I know this is not clear or documented, and it would be better to have a "value" argument in the constructor or somesuch.
