Using Umbraco 10.3.2 and tring to follow this documentation https://our.umbraco.com/documentation/Tutorials/creating-a-property-editor/ and setup a custom property. It comes up in the common properties list to add however on adding it I get this error.

Received an error from the server
An error occurred
Object reference not set to an instance of an object.

Exception Details
System.NullReferenceException, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e: Object reference not set to an instance of an object.
Stacktrace
at Umbraco.Cms.Core.PropertyEditors.ConfigurationEditor`1.ToConfigurationEditor(TConfiguration configuration)
   at Umbraco.Cms.Core.Models.Mapping.DataTypeMapDefinition.MapPreValues(IDataEditor source, MapperContext context)
   at Umbraco.Cms.Core.Mapping.UmbracoMapper.<>c__DisplayClass9_0`2.<Define>b__0(Object source, 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 Umbraco.Cms.Web.BackOffice.Controllers.DataTypeController.GetPreValues(String editorAlias, Int32 dataTypeId)
   at lambda_method1858(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|26_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)

Any suggestions