gracefully handle null values in BindingExpression.TryConvert

[thisisthekap]

Description of Change

Added null handling to BindingExpression.TryConvert to mitigate an NullReferenceException which thereafter leads to app crash in production.

Issues Resolved

• fixes [Bug] Crash with NullReferenceException in Xamarin.Forms.BindingExpression.TryConvert on iOS #15381

API Changes

None

Platforms Affected

• Core/XAML (all platforms)

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

None

PR Checklist

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

[jfversluis]

Hey @thisisthekap thanks for this PR! I'm kind of on the fence about this one. I see how this makes sense, on the other hand you might want to include that null check earlier in your own code?

But even more so, this is technically a breaking change since this changes the behavior of how it has always worked.

[StephaneDelcroix]

as it only happens from time to time in prod, and you can't reproduce, I can't say if the attempted fix will solve your problem, or if you won't have another issue later on. but the fix is safe, I don't see any reasons not to merge it

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jfversluis]

Verified on our non-public pipeline that this builds