Error when session runs out

This isn't really a problem, but since I use Radzen it always showed me an error in the browser-console when the session ran out:
ERROR TypeError: "items is null"

What could that be?

ERROR TypeError: "items is null"
Angular 3
    syncWithRouter
    sync
    childrenToItems
RxJS 6
    __tryOrUnsub
    next
    _next
    next
    _next
    next
schedulerFn Angular
RxJS 5
    __tryOrUnsub
    next
    _next
    next
    next
Angular 25
    emit
    notifyOnChanges
    checkAndUpdateQuery
    execQueriesAction
    checkAndUpdateView
    callViewAction
    execComponentViewsAction
    checkAndUpdateView
    callViewAction
    execEmbeddedViewsAction
    checkAndUpdateView
    callViewAction
    execComponentViewsAction
    checkAndUpdateView
    detectChanges
    tick
    tick
    next
    invoke
    onInvoke
    invoke
    run
    run
    next
    schedulerFn
RxJS 5
    __tryOrUnsub
    next
    _next
    next
    next
Angular 9
    emit
    checkStable
    onLeave
    onInvokeTask
    invokeTask
    runTask
    invokeTask
    invokeTask
    globalZoneAwareCallback
    core.js:15724
        Angular 4
        RxJS 5
        Angular 11

We don't have any idea what this issue could be. Not sure what items is etc. Looks like some Angular Router exception. Can't tell anything else from the provided info.