Check if Base property on tcImports indeed exists inside ProvidedTypesContext.Create

[Jand42]

I have encountered the very same error as described in #177 in a different context: checking a project using a TP with FCS running inside an ASP.NET application (both the web application and and the checked project are targeting .NET 4.6).

I have found that a null check on dllInfo value inside ProvidedTypesContext.Create before using it eliminates this error and I could not find any regression.

[dsyme]

@Jand42 Thanks!