Before you can handle this, you need to decide exactly how you intend to display a null OT or a null PT.
Right now, you are attempting to display a three-valued property with a two-valued control. In other words, there are THREE possible values for OT and PT: true, false, and NULL. However, there are only two states for a checkbox: checked and unchecked.
You would normally show a true value with a checked checkbox; and a false value with an unchecked checkbox. But, how do you intend to display a NULL value for OT or PT?
You could write code to show a NULL value as an unchecked checkbox, for example. But that way, there is now way to distinguish a NULL from a false when the values are displayed. Does that make sense?