title: Failed to call Activator.CreateInstance with GraphQL and .NET core
body: <p>When executing a query I get this error: <code>{"failed to call activator.createinstance. type: api.graphql.lessonquery"}</code></p>
  
  <p>The constructor of Lessonquery is as follows: </p>
  
  <pre><code>private readonly AppDbContext _db;
  
  public LessonQuery(AppDbContext dbContext)
      {
          _db = dbContext;
      }
  </code></pre>
  
  <p>And I have added <code>services.AddSingleton&lt;AppDbContext&gt;();</code> to the <code>ConfigureServices()</code> in startup.cs</p>
  
  <p>I include my LessonQuery class in the schema like this</p>
  
  <pre><code>public LessonSchema()
  {
      this._schema = Schema.For(@"
  
      type Lesson {
          id: ID,
          title: String,
          level: String,
          exercises : [String]
      }
  
      type Query {
          lessons: [Lesson],
          lesson(id: ID): [Lesson]
      }
  
  
  ", _ =&gt;
      {
          _.Types.Include&lt;LessonQuery&gt;();
  
      });
  </code></pre>
  
  <p>I am looking for a way to fix this error? </p>