chore: move the app and the tests to .NET 10 (#59) - #60

Non-nullable field '_text' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

Non-nullable field '_root' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

Possible null reference return.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Nullability of reference types in type of 'event EventHandler RelayCommand<T>.CanExecuteChanged' doesn't match implicitly implemented member 'event EventHandler? ICommand.CanExecuteChanged'.

Nullability of reference types in type of parameter 'parameter' of 'void RelayCommand<T>.Execute(object parameter)' doesn't match implicitly implemented member 'void ICommand.Execute(object? parameter)' (possibly because of nullability attributes).

Nullability of reference types in type of parameter 'parameter' of 'bool RelayCommand<T>.CanExecute(object parameter)' doesn't match implicitly implemented member 'bool ICommand.CanExecute(object? parameter)' (possibly because of nullability attributes).

Nullability of reference types in type of 'event PropertyChangedEventHandler CalculatorViewModel.PropertyChanged' doesn't match implicitly implemented member 'event PropertyChangedEventHandler? INotifyPropertyChanged.PropertyChanged'.