function _isFormValid(){
            if(elements.inputCategory.val().length === 0 ||
                elements.inputName.val().length === 0 ||
                elements.inputColor.val().length === 0){
                $yobi.alert(Messages("label.failedTo", Messages("label.add")) + "\n" + Messages("label.error.empty"));
                return false;
            }

            if(_getRefinedHexColor(elements.inputColor.val()) === false){
                $yobi.alert(Messages("label.failedTo", Messages("label.add")) + "\n" + Messages("label.error.color", elements.inputColor.val()));
                return false;
            }

            return true;
        }
Returns whether is form valid
and shows error if invalid.

@returns {boolean}
@private