Custom navigation pane causing content not found exception

Description

Steps to reproduce

1. Copy code from this blog: https://www.epinova.no/en/blog/custom-icontent-with-its-own-navigation-pane/
2. Browse the site.

Expected
Get a new tab in the assets.

Actual
A 500 error from the server when accessing a URL similar to this one:

http://localhost/EPiServer/cms/Stores/contentstructure/?references=-1_-1&query=getchildren&allLanguages=true&typeIdentifiers=cms11.business.test.review&sort(+name)&dojo.preventCache=1556270728019

ContentNotFoundException: Content with id -1_-1 was not found
    EPiServer.Core.Internal.DefaultContentLoader.Get(ContentReference contentLink, LoaderOptions loaderOptions) +530
    EPiServer.Cms.Shell.UI.Rest.ContentQuery.GetChildrenQuery.GetChildren(ContentQueryParameters parameters, ContentReference parentId, LanguageSelector selector) +233
    EPiServer.Cms.Shell.UI.Rest.ContentQuery.GetChildrenQuery.GetContent(ContentQueryParameters parameters) +562
    EPiServer.Cms.Shell.UI.Rest.ContentQuery.ContentQueryBase.ExecuteQuery(IQueryParameters parameters) +127
    EPiServer.Cms.Shell.UI.Rest.Internal.ContentStore.Get(ContentReference id, String query, ContentReference referenceId, ContentReference[] references, String[] typeIdentifiers, Nullable`1 allLanguages, IEnumerable`1 sortColumns, ItemRange range) +835
    lambda_method(Closure , ControllerBase , Object[] ) +486
    System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +229
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +35
    System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +39
    System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +77
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +42
    System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +72
    System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +387
    System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +387
    System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +387
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +42
    System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +38
    System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +188
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +38
    EPiServer.Shell.Services.Rest.RestControllerBase.EndExecute(IAsyncResult asyncResult) +122
    EPiServer.Shell.Services.Rest.RestHttpHandler.EndProcessRequest(IAsyncResult result) +108
    System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +602
    System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +195
    System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +128