Converter Class for Brazilian Cities

Gentlemen, I have two combos in a form, the first one being loaded with the Brazilian states, routinely I have passed the acronym of the chosen state to a routine in my control, which calls a routine in my persistence class, with the parameter.  The routine works (I see in the step-by-step), but at the time of loading the combo, it generates the following exception: "Validation error: value is required". I do not know how to make the class convert. Here is an excerpt from the code: