fix warnings in templates

Author: StephaneDelcroix

src/Templates/src/templates/maui-mobile/AppShell.xaml.cs

@@ -50,7 +50,7 @@ public static async Task DisplayToastAsync(string message)
 		await toast.Show(cts.Token);
 	}
 
-	private void SfSegmentedControl_SelectionChanged(object sender, Syncfusion.Maui.Toolkit.SegmentedControl.SelectionChangedEventArgs e)
+	private void SfSegmentedControl_SelectionChanged(object? sender, Syncfusion.Maui.Toolkit.SegmentedControl.SelectionChangedEventArgs e)
     {
 		Application.Current!.UserAppTheme = e.NewIndex == 0 ? AppTheme.Light : AppTheme.Dark;
     }

src/Templates/src/templates/maui-mobile/Pages/Controls/TaskView.xaml.cs

@@ -22,11 +22,11 @@ public ICommand TaskCompletedCommand
 		set => SetValue(TaskCompletedCommandProperty, value);
 	}
 
-	private void CheckBox_CheckedChanged(object sender, CheckedChangedEventArgs e)
+	private void CheckBox_CheckedChanged(object? sender, CheckedChangedEventArgs e)
 	{
-		var checkbox = (CheckBox)sender;
-		
-		if (checkbox.BindingContext is not ProjectTask task)
+		var checkbox = (CheckBox?)sender;
+
+		if (checkbox?.BindingContext is not ProjectTask task)
 			return;
 
 		if (task.IsCompleted == e.Value)