NullReferenceException breaks suggest for everything

Every use of `suggest get` fails, no matter the command. Even when asking about its own completions, it throws:

```
> dotnet suggest get dotnet-suggest
Unhandled exception: System.NullReferenceException: Object reference not set to an instance of an object.
at System.CommandLine.Suggest.SuggestionDispatcher.Get(InvocationContext context) in /_/src/System.CommandLine.Suggest/SuggestionDispatcher.cs:line 141
at System.CommandLine.Suggest.SuggestionDispatcher.<.ctor>b__2_2(InvocationContext context) in /_/src/System.CommandLine.Suggest/SuggestionDispatcher.cs:line 51
at System.CommandLine.Invocation.AnonymousCommandHandler.Invoke(InvocationContext context) in /_/src/System.CommandLine/Invocation/AnonymousCommandHandler.cs:line 24
at System.CommandLine.Invocation.AnonymousCommandHandler.InvokeAsync(InvocationContext context, CancellationToken cancellationToken) in /_/src/System.CommandLine/Invocation/AnonymousCommandHandler.cs:line 38
at System.CommandLine.Invocation.InvocationPipeline.InvokeAsync(ParseResult parseResult, IConsole console, CancellationToken cancellationToken) in /_/src/System.CommandLine/Invocation/InvocationPipeline.cs:line 31
```

This is with 1.1.415701+259d24fa2ed4d9114bc560b68c1bb319f4d6dd39