title: Exception parsing json with System.Text.Json.Serialization
body: <p>My sample code is very simple:</p>
  
  <pre><code>using System.Text.Json.Serialization;
  using Newtonsoft.Json;
  
  public class C {
    public C(string PracticeName) { this.PracticeName = PracticeName; }
    public string PracticeName;
  }
  
  var x = new C("1");
  var json = JsonConvert.SerializeObject(x); // returns "{\"PracticeName\":\"1\"}"
  
  var x1 = JsonConvert.DeserializeObject&lt;C&gt;(json); // correctly builds a C
  
  var x2 = System.Text.Json.Serialization.JsonSerializer.Parse&lt;C&gt;(json);
  </code></pre>
  
  <p>the last line raises:</p>
  
  <blockquote>
    <p>Exception thrown: 'System.NullReferenceException' in System.Text.Json.dll
    Object reference not set to an instance of an object.</p>
  </blockquote>
  
  <p>What am I doing wrong ?</p>
  
  <p>(Note this is on latest .NET Core 3 preview 5 with latest System.Text.Json 4.6.0-preview6.19259.10)</p>
  
  <p>Adding a parameterless constructor prevents the exception however I don't want/need a parameterless constructor and Json.Net parses fine without it.</p>
  
  <p>Is there a way to make System.Text.Json parse using the given constructor like Json.Net does ?</p>