Fix Entry crashing on iOS < 14

[rotorgames]

Description of Change

Fixing crashes on iOS < 14 if ClearButtonVisibility of Entry is WhileEditing

Issues Resolved

Fixes:

• [Bug] Xamarin.forms iOS Latest Stable Version Crash: System.NullReferenceException: Object reference not set to an instance of an object at Xamarin.Forms.Platform.iOS.EntryRendererBase`1[TControl].GetClearButtonTintImage #14479
• [Bug] Xamarin.Forms.Platform.iOS.EntryRendererBase`1[TControl].GetClearButtonTintImage crash #14510

API Changes

None

Platforms Affected

• iOS

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

• Use iOS simulator with iOS 13 or lower
• Set ClearButtonVisibility of Entry to WhileEditing
• The app must not crash when a page is opening

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[rotorgames]

@jfversluis Re-open this, please. Explanations: #14566 (comment)

[jfversluis]

Reopening this to make sure we have another look

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[VitalyKnyazev]

I am getting new exception on iOS 12.4 after updating to SR6:
Name: NSRangeException Reason: Cannot remove an observer ... for the key path "sublayers" from <CALayer 0x600007b11e80> because it is not registered as an observer.

It happens on EntryRenderer Dispose call at this line:
ClearButton?.Layer?.RemoveObserver(this, new NSString("sublayers"));

[lee-m]

@rotorgames @jfversluis I'm also seeing the same crash that @VitalyKnyazev reports above in my app after updating to SR6.

AddObserver is documented as returning "An IDisposable object.  Invoke the Dispose method on this object to remove the observer." so maybe we need to store the return value from AddObserver and dispose it instead of calling RemoveObserver?

EDIT: This was on an iPhone 6 running iOS 12.5.5, seems sensitive to the version of iOS being used much like the original issue was.

[jfversluis]

@VitalyKnyazev @lee-m would you mind opening a new issue for this and if possible attach a small reproduction sample to show the issue? That would be very helpful. If you can do that I will see to prioritize this for the next release

[lee-m]

@jfversluis @VitalyKnyazev I've opened #14804 for this. I've left the repro steps blank for now whilst I try and distill down a small example that demonstrates the problem but it might be a bit tricky.

[xleon]

I'm getting this crash too.
It doesn't matter what ClearButtonVisibility is set to. It crashes anyway when disposing (normally when navigating away from the page where the Entry is).
As a workaround I'm overriding Dispose() method with a try/catch in my custom EntryRenderer:

protected override void Dispose(bool disposing)
{
    try
    {
        base.Dispose(disposing);
    }
    catch (Exception)
    {
        // TODO remove this hack on newer version of Xamarin.Forms: https://github.com/xamarin/Xamarin.Forms/pull/14526
    }
}