Linking with Obj-C not working

I'm linking with an Objective-C lib, and everything compiles, but the app starts up and then quits with this Application Output:

    2014-12-08 14:41:55.846 iPhoneApp[4535:624418] Debug: Finished launching
    Debug: Finished launching
    2014-12-08 14:41:55.853 iPhoneApp[4535:624418] *** NSForwarding: warning: object 0x7b9529d0 of class 'FareInspector_Scanner' does not implement methodSignatureForSelector: -- trouble ahead
    2014-12-08 14:41:55.853 iPhoneApp[4535:624418] *** NSForwarding: warning: object 0x7b9529d0 of class 'FareInspector_Scanner' does not implement doesNotRecognizeSelector: -- abort

Here is the beginning of my Scanner class:

    namespace FareInspector {
        public class Scanner : xValidator {
            ...

... and here are its ancestors:

[BaseType(typeof(NSObject))]
[Model]
[Protocol]
public partial interface NofityUIDelegate {
    [Export("notifyUI:")]
    void NotifyUI(string sResult);
}

[BaseType(typeof(NSObject))]
public partial interface xValidator : NofityUIDelegate {
    ...

The posts I've found about this issue sound like its an inheritance problem, that NSObject isn't being used, but it is specified in what I have above. This was working the last time I opened the project a couple of months ago - I don't know if an iOS update, a Xamarin update, some settings change, or something else caused an issue, but I'm willing to try anything at this point. Please help me figure this one out.

Thanks, Craig