title: Error extracting metadata in docfx project
body: <p>Despite reading through the <a href="https://github.com/dotnet/docfx" rel="nofollow noreferrer">docfx documentation</a>, and numerous Github issues, I am unable to get my docfx project working.</p>
  <p>Steps taken:</p>
  <ol>
  <li>Initialise root directory <code>docfx init -q</code></li>
  <li>Build from root directory <code>docfx build</code> (no errors)</li>
  <li>Serve <code>docfx &quot;/user/docfx_project/docfx.json&quot; --serve</code></li>
  </ol>
  <p>Stacktrace:</p>
  <pre><code>Error:Error extracting metadata for /user/docfx_project/src/MyApp/MyApp.csproj,/user/docfx_project/src/MyApp.Tests/MyApp.Tests.csproj: System.NullReferenceException: Object reference not set to an instance of an object
    at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.SaveAllMembersFromCacheAsync () [0x00380] in &lt;0779a76130d64c11baa2c8c084cbfe7c&gt;:0 
    at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.ExtractMetadataAsync () [0x000c0] in &lt;0779a76130d64c11baa2c8c084cbfe7c&gt;:0 
  </code></pre>
  <p>Environment:</p>
  <ul>
  <li>OSX 10.13.6</li>
  <li>docfx 2.56.2.0</li>
  <li>.NET Core SDK (3.1.401)</li>
  <li>Mono JIT compiler version 6.10.0.104</li>
  </ul>
  <p>Project structure:</p>
  <pre><code>docfx-project
   |
   +-- _site
   +-- api
   +-- apidoc
   +-- articles
   +-- docfx.json
   +-- images
   +-- index.md
   +-- obj
   +-- src
       |
       +-- MyApp
           |
           +-- MyApp.csproj
   +-- toc.yml
  </code></pre>
  <p>docfx.json:</p>
  <pre><code>{
    &quot;metadata&quot;: [
      {
        &quot;src&quot;: [
          {
            &quot;files&quot;: [
              &quot;src/**/**.csproj&quot;
            ]
          }
        ],
        &quot;dest&quot;: &quot;api&quot;,
        &quot;disableGitFeatures&quot;: false,
        &quot;disableDefaultFilter&quot;: false
      }
    ],
    &quot;build&quot;: {
      &quot;content&quot;: [
        {
          &quot;files&quot;: [
            &quot;api/**.yml&quot;,
            &quot;api/index.md&quot;
          ]
        },
        {
          &quot;files&quot;: [
            &quot;articles/**.md&quot;,
            &quot;articles/**/toc.yml&quot;,
            &quot;toc.yml&quot;,
            &quot;*.md&quot;
          ]
        }
      ],
      &quot;resource&quot;: [
        {
          &quot;files&quot;: [
            &quot;images/**&quot;
          ]
        }
      ],
      &quot;overwrite&quot;: [
        {
          &quot;files&quot;: [
            &quot;apidoc/**.md&quot;
          ],
          &quot;exclude&quot;: [
            &quot;obj/**&quot;,
            &quot;_site/**&quot;
          ]
        }
      ],
      &quot;dest&quot;: &quot;_site&quot;,
      &quot;globalMetadataFiles&quot;: [],
      &quot;fileMetadataFiles&quot;: [],
      &quot;template&quot;: [
        &quot;default&quot;
      ],
      &quot;postProcessors&quot;: [],
      &quot;markdownEngineName&quot;: &quot;markdig&quot;,
      &quot;noLangKeyword&quot;: false,
      &quot;keepFileLink&quot;: false,
      &quot;cleanupCacheHistory&quot;: false,
      &quot;disableGitFeatures&quot;: false
    }
  }
  </code></pre>
  <p>Halp.</p>