AX Exchange error on setting UIViewController canDisplayBannerAds true

I am getting thousands of these messages in iOS with Xamarin.Forms:

AX Exchange error: Error Domain=Accessibility Code=0 "Remote service does not respond to _accessibilityMachPort" UserInfo=0x170860680 {NSLocalizedDescription=Remote service does not respond to _accessibilityMachPort}

This started when I added a custom page renderer to turn on iAd banner:

public override void ViewDidLoad ()
{
    base.ViewDidLoad ();

    this.SetValueForKey (NSNumber.FromBoolean(true), new NSString ("canDisplayBannerAds"));
}

The good news is that the test ad banner appears in both simulator and device. The bad news is that these error messages occur -- on device only -- at the rate of about 2500 messages per minute! The messages stop only when I comment out the SetValueForKey line. (I'm using key value coding because I don't see that Xamarin UIViewController has the canDisplayBannerAds property.)

These message have also been reported by several forum readers in iOS forum, subject Anybody Else Seeing Weirdness with MFMailComposeViewController?