Using MultiNodeTreePicker with an xpath query of $site//cardRepository in Umbraco 8 which was once working, has started to throw the following error

Failed to retrieve entity data for query $site//cardRepository

Looking at the logs, I'm seeing the following

Unhandled controller exception occurred for request '{RequestUrl}'
Umbraco.Core.Exceptions.PanicException: invalid item type
   at Umbraco.Web.PublishedCache.NuCache.PublishedContent.GetGetterById() in D:\a\1\s\src\Umbraco.Web\PublishedCache\NuCache\PublishedContent.cs:line 123
   at Umbraco.Web.PublishedCache.NuCache.PublishedContent.<get_ChildrenForAllCultures>d__57.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Umbraco.Web.PublishedCache.NuCache.PublishedContent.get_ChildIds() in D:\a\1\s\src\Umbraco.Web\PublishedCache\NuCache\PublishedContent.cs:line 361
   at Umbraco.Web.PublishedCache.NuCache.Navigable.NavigableContent.get_ChildIds() in D:\a\1\s\src\Umbraco.Web\PublishedCache\NuCache\Navigable\NavigableContent.cs:line 54
   at Umbraco.Core.Xml.XPath.NavigableNavigator.State.GetContentChildIds(Int32 maxDepth) in D:\a\1\s\src\Umbraco.Core\Xml\XPath\NavigableNavigator.cs:line 1196
   at Umbraco.Core.Xml.XPath.NavigableNavigator.MoveToFirstChildElement() in D:\a\1\s\src\Umbraco.Core\Xml\XPath\NavigableNavigator.cs:line 579
   at Umbraco.Core.Xml.XPath.NavigableNavigator.MoveToNext() in D:\a\1\s\src\Umbraco.Core\Xml\XPath\NavigableNavigator.cs:line 758
   at MS.Internal.Xml.XPath.XPathDescendantIterator.MoveNext()
   at MS.Internal.Xml.XPath.DescendantQuery.Advance()
   at MS.Internal.Xml.XPath.DocumentOrderQuery.Evaluate(XPathNodeIterator context)
   at System.Xml.XPath.XPathNavigator.Evaluate(XPathExpression expr, XPathNodeIterator context)
   at System.Xml.XPath.XPathNavigator.Evaluate(XPathExpression expr)
   at System.Xml.XPath.XPathNavigator.Select(XPathExpression expr)
   at System.Xml.XPath.XPathNavigator.Select(String xpath)
   at Umbraco.Core.Xml.XPathNavigatorExtensions.Select(XPathNavigator navigator, String expression, XPathVariable[] variables) in D:\a\1\s\src\Umbraco.Core\Xml\XPathNavigatorExtensions.cs:line 20
   at Umbraco.Web.PublishedCache.NuCache.ContentCache.GetSingleByXPath(Boolean preview, String xpath, XPathVariable[] vars) in D:\a\1\s\src\Umbraco.Web\PublishedCache\NuCache\ContentCache.cs:line 321
   at Umbraco.Web.PublishedCache.PublishedCacheBase.GetSingleByXPath(String xpath, XPathVariable[] vars) in D:\a\1\s\src\Umbraco.Web\PublishedCache\PublishedCacheBase.cs:line 51
   at Umbraco.Web.PublishedContentQuery.ContentSingleAtXPath(String xpath, XPathVariable[] vars) in D:\a\1\s\src\Umbraco.Web\PublishedContentQuery.cs:line 61
   at Umbraco.Web.Editors.EntityController.GetByQuery(String query, Int32 nodeContextId, UmbracoEntityTypes type) in D:\a\1\s\src\Umbraco.Web\Editors\EntityController.cs:line 267
   at lambda_method(Closure , Object , Object[] )
   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass6_2.<GetExecutor>b__2(Object instance, Object[] methodParameters)
   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute(Object instance, Object[] arguments)
   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__15.MoveNext()

Any help would be greatly appreciated.