title: Plugin Pre Operation Create - Update field error
body: <p>My plugin fire on Pre Create operation on Entity X. When trying to update a field on the Entity X using the following code I am getting error:</p>
  
  <pre><code>trEntity = (Entity)context.InputParameters["Target"];
  trGuid = (Guid)trEntity.Id;
  
  tr = (Entity)service.Retrieve("EntityX", trGuid,
                      new ColumnSet(new string[] { "field_a", "field_b" }));
  
  
  tr["field_a"] = null;
  service.Update(tr);
  </code></pre>
  
  <p>The error I am getting is: 
  <strong>Entity X with Id = 11505683-2292-b537-e311-143710e56fb7  Does Not Exist</strong></p>