Avoid NullReferenceException for MaterialActivityIndicatorRenderer

[tomeverin]

Under some conditions, possibly related to ActivityIndicator visibility, Control may be null at time of call to LayoutSubviews(). This caused crash due to NullReferenceException. This null check avoids that problem.

Note: this is the second PR for the same; was accidentally merging from master to 4.6.0 on the first one.

Description of Change

Null check for Control in Xamarin.Forms.Material.iOS.MaterialActivityIndicatorRenderer.LayoutSubviews()

Issues Resolved

I noticed that at times I was seeing crashes due to NullReferenceException following Shell's GoToAsync method. On further investigation it was due to attempted access on a null Control. This null check solved the problem.

API Changes

None.

Platforms Affected

• iOS only

Behavioral/Visual Changes

Resolution of NullReferenceException crashes

Before/After Screenshots

Not applicable

Testing Procedure

I wasn't able to come up with a simple repro, however the issue was reliably reproducible in my production app. It occurred on returning to a ShellItem via GoToAsync for a second time. May have something to do with ActivityIndicator visibility.

PR Checklist

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

[tomeverin]

Ping @PureWeen 🙏
Currently needing to build my own Visual.Material local nuget at each release, with the null check, just to avoid the crashes