When the helpNamespace property in helpProvider is blank after delete the previous saved URL, project exit from runtime automatically when press F1

[Liv-Goh]

.NET version

.NET 9.0 SDK build: 9.0.100-preview.7.24321.3

Did it work in .NET Framework?

Yes

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

Yes, this is a regression issue. Not reproduced on .NET 8.0

Issue description

Current Behavior (.NET 9.0): When the helpNamespace property in helpProvider is blank after delete the previous saved URL, project exit from runtime automatically when press F1 and no error exception popped out.

https://github.com/dotnet/winforms/assets/169347065/fb053321-b64c-4e64-ab81-9c72b4d5ffcf

Expected Behavior (.NET 8.0): When the helpNamespace property in helpProvider is blank after delete the previous saved URL, error exception popped out when press F1.

Steps to reproduce

1. Create a WinForms .NET 9.0 project
2. Add a button and helpProvider to form designer
3. Set button "showHelpOnHelpProvider1" property to True
4. Add a random URL (e.g. https://github.com/dotnet/winforms) to helpProvider "HelpNamespace" property, save
5. Delete URL in "HelpNamespace", save
6. Run the project and press F1