Refactor Logic into a ViewModelBase Class

public class MainViewModel : INotifyPropertyChanged { … }

public class ViewModelBase : INotifyPropertyChanged
{
    // Moved from MainViewModel:
    public event PropertyChangedEventHandler? PropertyChanged;

    // Moved from MainViewModel:
    // Change access modifier to protected to call this from subclasses:
    protected virtual void RaisePropertyChanged(string? propertyName = null)
    {
        // The sender is this MainViewModel instance;
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

public class MainViewModel : ViewModelBase { … }