title: Dotnet Core 3.1 Upgrade error - Method 'GetValidationVisitor' does not have an implementation
body: <p>I have upgraded WEB API dotnet core 2.2 project to 3.1.</p>
  <p>inside the <code>Startup.CS</code> file I have the following method :</p>
  <pre><code>    public static IServiceCollection RegisterMvc(this IServiceCollection services, IConfiguration configuration, IWebHostEnvironment environment)
      {
          try
          {
              services
                  .AddMvc(options =&gt;
                  {
                       options.Filters.Add(typeof(ValidationActionFilter));
                  })
                  .AddControllersAsServices()
                  .AddNewtonsoftJson(options =&gt;
                  {
                      options.SerializerSettings.NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore;
                      options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
                  })
                  .AddFluentValidation(fv =&gt;
                  {
                      fv.ImplicitlyValidateChildProperties = true;
                      fv.RegisterValidatorsFromAssemblyContaining&lt;MyValidationClass&gt;();
                  });
          }
          catch (ReflectionTypeLoadException ex)
          {
              foreach (Exception inner in ex.LoaderExceptions)
              {
                  Console.WriteLine(inner.Message);
              }
              throw ex;
          }
          return services;
      }
  </code></pre>
  <p>Later I use this <code>RegisterMvc()</code> method inside <code>ConfigureServices()</code> to do the dependency injection.</p>
  <p>After upgrading to the new dotnet core, I am getting the following error, and I am clueless about what it's trying to say or how to resolve it:</p>
  <pre><code>System.Reflection.ReflectionTypeLoadException: 'Unable to load one or more of the requested types.
  Method 'GetValidationVisitor' in type    
  'FluentValidation.AspNetCore.FluentValidationObjectModelValidator' from assembly  
  'FluentValidation.AspNetCore, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7de548da2fbae0f0' 
   does not have an implementation.'
  </code></pre>