CancellationTokenSource disposed in AbstractExtensionMethodImportCompletionProvider.cs

[KirillOsenkov]

Version Used:
17.0.0

Steps to Reproduce:

1. Use a lightbulb for importing extension methods

Under unknown conditions we get an exception:

System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at void System.Threading.CancellationTokenSource.ThrowObjectDisposedException()
   at Task Microsoft.CodeAnalysis.Completion.Providers.AbstractExtensionMethodImportCompletionProvider.AddCompletionItemsAsync(CompletionContext completionContext, SyntaxContext syntaxContext, HashSet<string> namespaceInScope, bool isExpandedCompletion, CancellationToken cancellationToken)+() => { } in C:/Roslyn/src/Features/Core/Portable/Completion/Providers/ImportCompletionProvider/AbstractExtensionMethodImportCompletionProvider.cs:line 50
   at bool System.Threading.ThreadPoolWorkQueue.Dispatch()

@genlu