Value cannot be null. (Parameter 'rootComponent') when use IRootDesigner

• .NET Core Version: .NET 5 updated with VS2019 (Version 16.8.1) community build. net5.0-windows

• Have you experienced this same bug with .NET Framework?: No

Problem description:

When I create the IRootDesigner myself component, open the deisnger of the component in VS 2019, it shows the error below.

Value cannot be null.  (Parameter 'rootComponent') 
Call Stack
at Microsoft.VisualStudio.WinForms.Protocol.Endpoints.Sessions.InitializeRootComponentResponse..ctor(IComponent rootComponent, IComponent[] components, String[] componentNames)
at WinFormsSurface.Handlers.Sessions.InitializeRootComponentHandler.HandleRequest(InitializeRootComponentRequest request)
at WinFormsSurface.Requests.RequestHandler`2.WinFormsSurface.Requests.IRequestHandler.HandleRequest(Request request) 

Then I create a sample from https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.design.irootdesigner?view=net-5.0 and open the deisgner of RootDesignedComponent it shows the same error!

The proejct use net5.0-windows as TargetFramework. and the Form1 inherit System.Windows.Forms.Form works fine but not the component!

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0-windows</TargetFramework>
    <UseWindowsForms>true</UseWindowsForms>
  </PropertyGroup>

</Project>

[safern]

Thanks all for the info. I was able to reproduce this. Taking a deeper look now.

[ATECoder]

I am seeing the exact same behavior. It is very easy to reproduce: (1) Add a new user control (use default name usercontrol1); (2) Hit Shift-F7–user control 1 displays in design mode; so far so good; (3) change public partial class UserControl1 : UserControl to public partial class UserControl1 : SplitContainer (2) Hit Shift-F7–error: Value cannot be null. (Parameter 'rootComponent')