An error dialog pops up if adding new item to project when form designer is "waiting for IntelliSense to finish initializing"

[Olina-Zhang]

.NET version

.NET 10 SDK build: 10.0.100-alpha.1.24508.48

Did it work in .NET Framework?

Yes

Did it work in any of the earlier releases of .NET Core or .NET 5+?

Yes, just repro after installed .NET 10 SDK build to create a .NET 9.0 app

Issue description

After installed .NET 10 SDK build from main branch, an error dialog pops up if adding new item: user control/form to project when form designer shows "waiting for IntelliSense to finish initializing". If close this error dialog, the item is added successfully, and build app without any error.

https://github.com/user-attachments/assets/363d3af9-7d7f-4123-9608-4dfe41a2c4cd

If form designer is fully loaded, then to add new item, this issue cannot repro. If uninstall .NET 10 SDK, this issue doesn't repro.

Steps to reproduce

0. Install .NET 10 SDK
1. Create a Winforms app with targeting .NET 9.0 in a new instance
2. When form designer shows "waiting for intellisense to finish initializing", Ctrl+Shift+A to add a new item: usercontrol/form quickly