Model Builder Error: Not implemented (Exception from HRESULT: 0x80004001 (E_NOTIMPL)) when adding project templates to solution where has one unloaded project.

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**System Information (please complete the following information):**
- Windows OS: Windows-10-Enterprise-21H1
- Microsoft Visual Studio Enterprise 2019: 16.11.4
- ML.Net Model Builder: 16.7.7.2150601 (Main Branch)

**Describe the bug**
- On which step of the process did you run into an issue: adding project templates to solution where has one unloaded project.
- Clear description of the problem: If there is an unload project in the solution, will prompt Model Builder Error when adding project on consume page. Reload the project, then can continue adding projects without any error.

**To Reproduce**
Steps to reproduce the behavior:
1. Select Create a new project from the Visual Studio 2019 start window;
2. Choose the C# Console App (.NET Core) project template with .Net 5.0;
3. Add model builder by right click on the project;
4. Select Data classification scenario to complete training;
5. Navigate to Consume page, add one ConsoleApp project;
6. Right click the generated ConsoleApp project to unload it;
7. Back to Consume page to continue adding other project templates;
8. Prompt Model Builder Error: Model Builder Error: Not implemented (Exception from HRESULT: 0x80004001 (E_NOTIMPL)).

**Expected behavior**
No error when adding project templates on consume page.

**Screenshots**
If applicable, add screenshots to help explain your problem.

Riproduci l’errore dalla pagina Consume in Visual Studio 2019 seguendo i passaggi elencati, soprattutto dopo aver scaricato il progetto ConsoleApp generato. Traccia il flusso di aggiunta dei modelli di progetto e la relativa gestione dei progetti scaricati; il lavoro è completato quando è possibile aggiungere altri modelli senza l’errore E_NOTIMPL.

Scritto dal modello di indicizzazione a partire dal testo della issue.