title: How can i fix the mscorlib error?
body: <p>I have been getting this kind of error on my project which has cost me valuable time. I don't know what to do at this moment. I have tried reference <code>MScorlib</code> with a higher version and the problem persist. Please look at my stack trace.</p>
  
  <blockquote>
    <p>System.NullReferenceException : Object reference not set to an instance of an object.
       at Microsoft.VisualStudio.LanguageServices.Implementation.VisualStudioAddMetadataReferenceCodeActionOperationFactoryWorkspaceService.AddMetadataReferenceOperation.Apply(Workspace workspace,CancellationToken cancellationToken)
       at async Microsoft.CodeAnalysis.Editor.Implementation.CodeActions.CodeActionEditHandlerService.ProcessOperationsAsync()
       at async Microsoft.CodeAnalysis.Editor.Implementation.CodeActions.CodeActionEditHandlerService.ApplyAsync()
       at async Microsoft.CodeAnalysis.Editor.Implementation.Suggestions.SuggestedAction.InvokeWorkerAsync()
       at async Microsoft.CodeAnalysis.Editor.Implementation.Suggestions.SuggestedAction.&lt;>c__DisplayClass19_0.b__0()
       at async Microsoft.CodeAnalysis.Extensions.IExtensionManagerExtensions.PerformActionAsync()</p>
  </blockquote>
  
  <p>The original error says </p>
  
  <blockquote>
    <p>you must reference mscorlib version 4.0.0.0, culture=natural,
    puclickeytoken=etc.</p>
  </blockquote>
  
  <p>How do I solve this?</p>