Crash in custom renderer on iOS

I have a custom renderer for a DatePicker. However the OnElementPropertyChanged event is being fired during Dispose of the view on iOS (doesn't seem to happen on the Android build), causing a segfault (null pointer dereference).

    public class OptionalDatePickerRenderer : DatePickerRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<DatePicker> e)
        {
            base.OnElementChanged(e);
            this.Control.Placeholder = "Date of Birth";
            SetText();
        }

        protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);
            if (e.PropertyName == "Date" || e.PropertyName == DatePicker.FormatProperty.PropertyName)
            {
                SetText();
            }
        }

        void SetText()
        {
            // date currently set on the optional date picker (date known to model)
            var date = Element.Date;
            if (date == Element.MinimumDate)
                Control.Text = "";
        }
    }

What should I be doing differently?