Installing and setting up SEOChecker v9 on a site migrated from Umbraco v8 works fine. But when you vist any node that contains the SEOChecker datatypes, this error is thrown.

SEOChecker was also installed on the original site before it was migrated. But all the old datatypes has been deleted, and the new datatypes has different names.

It hard to figure out when it says it cannot find content type with alias, and there is no alias.

Value cannot be null. (Parameter 'Cannot find a content type with alias ')
System.ArgumentNullException, System.Private.CoreLib, Version=5.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e: Value cannot be null. (Parameter 'Cannot find a content type with alias ') 
at SEOChecker.Common.Repository.Definition.DefinitionFactory.Single(String alias)

   at SEOChecker.Common.Helpers.ConfigurationHelper.GetDocumentTypeByAlias(SEOCheckerConfiguration configuration, String alias, String doctypeAlias)

   at SEOChecker.Common.Helpers.ConfigurationHelper.GetDocumentTypeByAlias(SEOCheckerConfiguration configuration, String alias)

   at SEOChecker.Common.PropertyEditors.DataValueEditors.RobotsTxtDataValueEditor.ToEditor(IProperty property, String culture, String segment)

   at Umbraco.Cms.Core.Models.Mapping.ContentPropertyBasicMapper`1.Map(IProperty property, TDestination dest, MapperContext context)

   at Umbraco.Cms.Core.Models.Mapping.ContentPropertyDisplayMapper.Map(IProperty originalProp, ContentPropertyDisplay dest, MapperContext context)

   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, Type sourceType, MapperContext context)

   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TSource,TTarget](TSource source, MapperContext context)

   at System.Linq.Enumerable.SelectIPartitionIterator`2.ToList()

   at Umbraco.Cms.Core.Models.Mapping.TabsAndPropertiesMapper.MapProperties(IContentBase content, List`1 properties, MapperContext context)

   at Umbraco.Cms.Core.Models.Mapping.TabsAndPropertiesMapper`1.Map(TSource source, MapperContext context)

   at Umbraco.Cms.Web.BackOffice.Mapping.ContentMapDefinition.Map(IContent source, ContentVariantDisplay target, MapperContext context)

   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, Type sourceType, MapperContext context)

   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, MapperContext context)

   at Umbraco.Cms.Core.Mapping.MapperContext.Map[TTarget](Object source)

   at Umbraco.Cms.Core.Models.Mapping.ContentVariantMapper.Map(IContent source, MapperContext context)

   at Umbraco.Cms.Web.BackOffice.Mapping.ContentMapDefinition.Map(IContent source, ContentItemDisplay target, MapperContext context)

   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, Type sourceType, MapperContext context)

   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, MapperContext context)

   at Umbraco.Cms.Web.BackOffice.Controllers.ContentController.MapToDisplay(IContent content, Action`1 contextOptions)

   at Umbraco.Cms.Web.BackOffice.Controllers.ContentController.MapToDisplay(IContent content)

   at Umbraco.Cms.Web.BackOffice.Controllers.ContentController.GetById(Int32 id)

   at lambda_method1061(Closure , Object , Object[] )

   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)

   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeActionMethodAsync()

   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)

   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeNextActionFilterAsync()

--- End of stack trace from previous location ---

   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)

   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)

   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()

--- End of stack trace from previous location ---

   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextExceptionFilterAsync>g__Awaited|25_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)