NullReferenceException after update to Xamarin.iOS 6.4.4.12

After updating Xamarin.iOS to 6.4.4.12 our code crashes with NullReferenceException when the event handler is unsubscribed from the event. The issue is reproduced only with Action event types and only during debugging. Deploy on simulator or device works without the exception. Rollback to 6.4.3 fixes this issue.

The example code causing the exception:

public class Test
{
    private event Action<string> testEvent;

    public Test()
    {
        testEvent += HandleTestEvent;
        testEvent -= HandleTestEvent; // NullReferenceException
    }

    private void HandleTestEvent(string text)
    {
    }
}