First, I don't think what you're doing is intended, exactly.  For a boolean property, why not just use tokens? The activity property binding functionality was intended for complex objects, such as a Laserfiche Entry or collections of things.

But to answer your posted question, if I recall correctly (and it's been a long time so take this with a grain of salt...), using the "reference to another activity" functionality requires that your runtime activity has a DependencyProperty. In this case, a DependencyProperty named UseAnchorStringProperty (the property name + "Property").  Something is assuming that property exists and it's throwing an unhandled exception when it doesn't.

For example, for custom activities that have a connection profile, our CustomProfileActivity has a ProfileId property declared like this:

        /// <summary>The name of the connection profile id property.</summary>
        public const string ProfileIdPropertyName = "ProfileId";

        /// <summary>The connection profile id property.</summary>
        public static DependencyProperty ProfileIdProperty;

        /// <summary>
        /// Initializes the <see cref="CustomProfileActivity"/> class.
        /// </summary>
        static CustomProfileActivity()
        {
            ProfileIdProperty = DependencyProperty.Register(ProfileIdPropertyName, typeof(Guid), typeof(CustomProfileActivity), new PropertyMetadata(DependencyPropertyOptions.Metadata));
        }