C# language features are not working

This was working at some point in the past without any project configuration changes since then.

Getting this exthost error:
```
[2018-12-15 11:02:48.220] [exthost] [error] [ms-vscode.csharp] provider FAILED
[2018-12-15 11:02:48.220] [exthost] [error] "System.NullReferenceException: Object reference not set to an instance of an object.\r\n at OmniSharp.Cake.Extensions.ResponseExtensions.d__7.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at OmniSharp.Cake.Services.RequestHandlers.CakeRequestHandler`2.d__14.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at OmniSharp.Endpoint.EndpointHandler`2.d__18.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at OmniSharp.Endpoint.EndpointHandler`2.d__16.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at OmniSharp.Stdio.Host.d__13.MoveNext()"
```

Version: 1.31.0-insider (user setup)
Commit: ece3a90fb58986570d8afdfb16530c4dd8c743af
Date: 2018-12-14T06:19:42.976Z
Electron: 3.0.10
Chrome: 66.0.3359.181
Node.js: 10.2.0
V8: 6.6.346.32
OS: Windows_NT x64 10.0.17134

Thanks!

The report names no repository file or test to start from; reproduce the C# language-feature failure using the listed VS Code and Windows versions and trace the OmniSharp.Cake.Services.RequestHandlers.CakeRequestHandler path. Use the ResponseExtensions.TranslateAsync stack frame to locate the failing request, and consider the issue done when C# features work without the reported provider failure.

Written by the indexing model from the issue text.