TodoApp.Uno: nullable reference warnings in sample code (CS8618, CS8622)

[adrianhall]

Summary

Now that #525/#524 (PR #533) let TodoApp.Uno build on every head in CI, the following pre-existing nullable-reference-type warnings are visible on every head (android, ios-maccatalyst, windows, browserwasm, desktop):

App.xaml.cs(15,12): warning CS8618: Non-nullable field 'dbConnection' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

Database/OfflineClientEntity.cs(16,19): warning CS8618: Non-nullable property 'Id' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Views/TodoListPage.xaml.cs(40,46): warning CS8622: Nullability of reference types in type of parameter 'sender' of 'void TodoListPage.PublishNotification(object sender, NotificationEventArgs args)' doesn't match the target delegate 'EventHandler<NotificationEventArgs>' (possibly because of nullability attributes).

CI run: https://github.com/CommunityToolkit/Datasync/actions/runs/29093946725 (all todoapp-uno / * jobs)

Root cause

Genuine, pre-existing nullable-reference-type gaps in the sample's own code, unrelated to the Uno Platform version or any change in PR #533:

• App.xaml.cs: dbConnection field is declared non-nullable but never initialized in the constructor.
• Database/OfflineClientEntity.cs: Id property is declared non-nullable but not set in the constructor / not marked required.
• Views/TodoListPage.xaml.cs: PublishNotification(object sender, NotificationEventArgs args) has non-nullable sender/args parameters that don't match the nullable annotations on the standard EventHandler<NotificationEventArgs> delegate it's assigned to (via ViewModel.NotificationHandler += PublishNotification;).

Suggested fix

• App.xaml.cs: add the required modifier to dbConnection, or initialize it, or declare it nullable if genuinely optional at construction time.
• Database/OfflineClientEntity.cs: add required to the Id property (mirroring the fix pattern already used elsewhere in this codebase for the same warning class).
• Views/TodoListPage.xaml.cs: change PublishNotification's signature to object? sender, NotificationEventArgs args (or whatever matches EventHandler<NotificationEventArgs>'s actual nullable annotations) to eliminate the delegate mismatch.

None of these require behavioral changes - just nullability annotation fixes.