Why can't I reactivate a view after I deleted it?

[xujiaguan]

this is my vm

  public partial class MainWindowViewModel : ViewModelBase
  {

 

      private IFactory? Factory;

      [ObservableProperty]
      private IRootDock? _layout;

      public MainWindowViewModel()
      {
          Factory = new Factory(CreateLayout);
          Layout = Factory.CreateLayout();
          Factory.InitLayout(layout: Layout);
      }


      private  IRootDock CreateLayout()
      {
          var document = new Document { Id = "Document1", Title = "Document 1" };
          var tool = new Tool { Id = "Tool1", Title = "Tool 1" };
          var documentDock = new DocumentDock
          {
              Id = "Documents",
              Title = "Documents",
              VisibleDockables = new List<IDockable> { document },
              ActiveDockable = document
          };

          var toolDock = new ToolDock
          {
              Id = "Tools",
              Title = "Tools",
              VisibleDockables = new List<IDockable> { tool },
              ActiveDockable = tool,
              Alignment = Alignment.Left
          };

          var mainLayout = new ProportionalDock
          {
              Id = "MainLayout",
              Orientation = Orientation.Horizontal,
              VisibleDockables = new List<IDockable>
          {
              toolDock,
              new ProportionalDockSplitter { CanResize = true },
              documentDock
          }
          };

          toolDock.Proportion = 0.25;
          documentDock.Proportion = 0.75;

          var rootDock = new RootDock
          {
              Id = "Root",
              Title = "Root",
              VisibleDockables = new List<IDockable> { mainLayout },
              ActiveDockable = mainLayout
          };

          return rootDock;
      }



      [RelayCommand]
      private void ActiveDocuments1()
      {
          Factory.RestoreDockable("Document1");
          
      }

      [RelayCommand]
      private void ActiveDocument2()
      {
          var c = Factory.RestoreDockable("Tool1");
      }

  }

When I close a document or tool and click to restore it, its return value is null, indicating that the view cannot be found.

this is my view

	<Grid RowDefinitions="100,*">
		<StackPanel>
			<Button Command="{Binding ActiveDocuments1Command}" Content="Add Document" Width="100" Height="25" Margin="5"/>
			<Button Command="{Binding ActiveDocument2Command}" Content="Add Tool" Width="100" Height="25" Margin="5"/>
		</StackPanel>
		<DockControl Grid.Row="1" Name="dc" Layout="{Binding Layout,Mode=TwoWay}" >
		</DockControl>
		
	</Grid>