[iOS] Fix NRE in ViewRenderer.LayoutSubviews

[rjantz3]

Description of Change

Protect against Element being null and causing trouble like it did in #13136.

API Changes

Changed:

• if(Control != null) => if(Control != null && Element != null)

Platforms Affected

• iOS

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

I was not able to duplicate this locally but my clients could repeatedly duplicate it on certain devices. After pushing this change to them the crashes stopped.

PR Checklist

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

[rjantz3]

This PR fixes a crash that gave me a stacktrace of the following:

ViewRenderer`2[TView,TNativeView].LayoutSubviews ()
(wrapper managed-to-native) UIKit.UIApplication.UIApplicationMain(int,string[],intptr,intptr)
UIApplication.Main (System.String[] , System.IntPtr , System.IntPtr )
UIApplication.Main (System.String[] , System.String , System.String )
Application.Main (System.String[] args)

[rmarinho]

/azp run

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