Crash when opening C# NET Core demos

[bernardnormier]

I got the following crash after opening the C# NET Core demos solution:

That's with VS2017 15.9.6 and Ice Builder 6.0.0.

[pepone]

I think the call here to ensure the project is checkout is not safe, as we don't have a project lock, seems the projects gets unloaded by other extension probably NuGet, as it typically load and unload projects during restoring.

ice-builder-visualstudio/IceBuilder_Common/ProjectHelperI.cs

Line 55 in d8f490b

project.EnsureIsCheckout();