Dereference of a possibly null reference.

Cannot convert null literal to non-nullable reference type.

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

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

Dereference of a possibly null reference.

Cannot convert null literal to non-nullable reference type.

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

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