C-Sharp WPF Binding TextBox.Text und Property MVVM (die 2.)

<TextBox x:Name="textBox1"  
	Text="{Binding Path= SourcePath.FolderPath, Mode= TwoWay, UpdateSourceTrigger= PropertyChanged}"/>  

<Button x:Name="button1"  
	Width="32"  
	Height="23"  
	Margin="623,117,0,0"  
	HorizontalAlignment="Left"  
	VerticalAlignment="Top"  
	Click="button_PrerequisitesPath_Click"  
	Content=". . ." />  

NameSpace MyNameSpace
{
    public class ViewModelBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        public void NotifyPropertyChanged(string propertyName, bool force)
        {
            if (force)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            else
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public class SourcePath : ViewModelBase
    {
        private static string _FolderPath;
        public string FolderPath
        {
            get { return _FolderPath; }
            set
            {
                if (_FolderPath == value)
                    return;

                _FolderPath = value;
                NotifyPropertyChanged("Prerequisites_FolderPath", false);  
            }
        }
    }
}


public partial class MainWindow : Window
{
  private void button1_Click(object sender, RoutedEventArgs e)
  {
      BrowseFolder();
  }

  private void BrowseFolder()
  {
      string toReturn = string.Empty;
      using (System.Windows.Forms.FolderBrowserDialog dialog = new System.Windows.Forms.FolderBrowserDialog())
      {
          SourcePath prereq = new SourcePath();
          dialog.ShowNewFolderButton = false;
          if (!string.IsNullOrEmpty(prereq.FolderPath))
              dialog.SelectedPath = prereq.FolderPath;
          else
              dialog.RootFolder = Environment.SpecialFolder.MyComputer;
          try
          {
              if (dialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
              {
                  prereq.FolderPath = dialog.SelectedPath;
              }
          }
          catch (Exception) { }
      }
  }
}