title: Swashbuckle.AspNetCore: 'No operations defined in spec!' problem after update of 'Microsoft.AspNetCore.Mvc.ApiExplorer' package to 2.2.0
body: <p>we have .net core 2.1 mvc webapi project which uses Swagger.
  we use following packages:</p>
  
  <pre><code>&lt;PackageReference Include="Swashbuckle.AspNetCore" Version="3.0.0" /&gt;
  &lt;PackageReference Include="Swashbuckle.AspNetCore.Annotations" Version="3.0.0" /&gt;
  &lt;PackageReference Include="Swashbuckle.AspNetCore.Filters" Version="4.3.0" /&gt;
  &lt;PackageReference Include="Microsoft.AspNetCore.Mvc.ApiExplorer" Version="2.1.2" /&gt;
  &lt;PackageReference Include="Microsoft.AspNetCore.Mvc.Versioning.ApiExplorer" Version="2.2.0" /&gt;
  &lt;PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="2.1.1" /&gt;
  </code></pre>
  
  <p>swashbuckle is configured in following way:</p>
  
  <pre><code>services.AddMvcCore().AddVersionedApiExplorer();
  services.AddApiVersioning();
  services.AddSwaggerGen();
  </code></pre>
  
  <p>everything woks in such setup (<code>/swagger/v1/swagger.json</code> has all operations and definitions and UI is rendered properly -> having all controllers and actions, etc)</p>
  
  <p>we are trying to migrate our .net core project from 2.1 to 2.2 .net core.</p>
  
  <p>In order to do that (without warnings) we need upgrade <code>Microsoft.AspNetCore.Mvc.ApiExplorer</code> nuget from 2.1.2 to 2.2.0.</p>
  
  <p>After this nuget update swagger.json (<code>/swagger/v1/swagger.json</code>) doesn't contain any <code>"paths": {}</code> and <code>"definitions": {}</code> and this results in swagger UI showing no controllers/actions (it renders: <code>No operations defined in spec!</code></p>
  
  <p>after upgrade package these package versions is updated:</p>
  
  <pre><code>&lt;PackageReference Include="Microsoft.AspNetCore.Mvc.ApiExplorer" Version="2.2.0" /&gt; //was 2.1.2
  &lt;PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="2.2.0" /&gt; //was 2.1.1
  </code></pre>
  
  <p>I've tried to upgrade swashbuckle version to v4.0.0 but it did not resolved the issue.</p>
  
  <p>what im missing which packages needs to be updated also so <code>swagger.json</code> would be generated properly when using <code>Microsoft.AspNetCore.Mvc.ApiExplorer 2.2.0</code> package ?
  or i'm missing something else ?</p>