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Created attachment 1560 [details]
The documentation for cancelTrackingWithEvent: indicates that nil is an acceptable argument, and this does in fact work (and is useful). The binding for UIControl.CancelTracking does not reflect this. It throws an ArgumentNullException if you give it a null event.
Attached is a sample project to demonstrate the exception and to demonstrate that you can call cancelTrackingWithEvent: with nil. If you long press the button that says "Works" then it just cancels the tracking (the button unhighlights). If you do the same with the "Broken" button then the app crashes.
Another workaround aside from calling directly (as I did in my example project) is to use CancelTracking(new UIEvent()) rather than CancelTracking(null). It's better than using void_objc_sendMesg_IntPtr, but it's not as good as just passing null.
Fixed in master (fc56dd1f) and 5.2-series (ada3c170).
It will be included in 5.3.3 and 5.2.11.