Support Nullable Bindable Properties in TypedBinding.cs

[JoeVoo]

Remove the null value Exception

• Bug fix

Description of Change

Remove the exception fired on null value that is not needed
var value = GetTargetValue(target.GetValue(property), typeof(TProperty)) ?? throw new InvalidOperationException("Unable to find target value");

Linked Issues

• Fixes [BUG] Set nullable BindableProperty to null throws Exception #354

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

• null values must be allowed here

[bijington]

I wonder if we should check whether TProperty is nullable in this scenario. Thoughts @TheCodeTraveler?

[TheCodeTraveler]

Thanks @JoeVoo! Could you please add unit tests that demonstrate the bug you are fixing and that verify the fix in this PR? It'd be great if you could also update the sample app to include a demonstration of how users were impacted; we use the sample app as a regression test against new PRs to ensure they don't break any existing behavior.

I've re-added the PR Checklist from our Pull Request Template to this PR Description that contains this checklist.

[JoeVoo]

@dotnet-policy-service agree

[JoeVoo]

Nullable reference types are enabled in the Unit Tests. I think this happens only in a 'nullable ref type disabled' project. But i am no test specialist. I try to avoid using enable.
My control that got the exception is a generic localized enum picker that's selectedValue can be bound and set null.

public partial class EnumPicker<T> : Picker where T : struct, Enum 
...
       public static readonly BindableProperty SelectedValueProperty =
           BindableProperty.Create(
           nameof(SelectedValue),
           typeof(T?),
           typeof(EnumPicker<T>),
           null,
           BindingMode.TwoWay,
           null,
           propertyChanged: SelectedValue_Changed);

typeof(T?) and EnumOfT is not nice, but the only way i found to set it null

[JoeVoo]

@bijington , what is your scenario to get the exception?

[bijington]

I am not experiencing this issue but I would be happy to try and contribute a unit test which would highlight the issue and fix if that helps?

[TheCodeTraveler]

Closing in favor of #381; same solution with unit tests included