Nullreference on popmodal and custom iOS renderer - xamarin

Question

I have a modal view, in which I have multiple Entry fields that I through an iOS customrenderer have customized to change BorderColor when Focused.
When i pop my modal view on button press:
await Navigation.PopModalAsync(true);
I get a nullreference in my iOS customrenderer, because I guess the element suddently becomes null, and i somehow haven't told it, that the view is gone.
public class BorderColorChange : EntryRenderer
{
protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
{
base.OnElementChanged(e);
if (Control != null)
{
Control.Layer.BorderWidth = 1;
Control.Layer.CornerRadius = 4;
e.NewElement.Focused += (sender, evt) =>
{
Control.Layer.BorderColor = UIColor.FromRGB(3, 169, 244).CGColor;
};
e.NewElement.Unfocused += (sender, evt) =>
{
Control.Layer.BorderColor = UIColor.LightGray.CGColor;
};
};
}
}
I've noticed, that when i remove the await keyword from Navigation.PopModalAsync(true); it doesn't produce the error.
Any help on how to solve this error?

Answer 1

It is perfectly normal for OnElementChanged to be called with e.NewElement==null. This just means that the element is being removed (like when you await the PopModelAsync), so it should handle the change that the new element to associate with is null.
With custom renderers, you need to both subscribe and unsubscribe to events when changes occur in associating your custom renderer with a native control. So for example:
public class BorderColorChange : EntryRenderer
{
private void MyFocusedEventHandler(...) ...
private void MyUnfocusedEventHandler(...) ...
protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
{
base.OnElementChanged(e);
if (Control != null)
{
Control.Layer.BorderWidth = 1;
Control.Layer.CornerRadius = 4;
if (e.OldElement != null) // unsubscribe from events on old element
{
e.OldElement.Focused -= MyFocusedEventHandler;
e.OldElement.Unfocused -= MyUnfocusedEventHandler;
}
if (e.NewElement != null) // subscribe to events on new element
{
e.NewElement.Focused += MyFocusedEventHandler;
e.NewElement.Unfocused += MyUnfocusedEventHandler;
}
}
}
}
The logic for what to do when the entry gets/loses focus goes into the MyFocusedEventHandler/MyUnfocusedEventHandler rather than inline to allow for both subscribing and unsubscribing.