Hi Andrew,

Thanks for the question. The error is because, exactly, that you are using a MultiPageViewModel. If you would kindly take a look at the MVVMSamples shipped with Crosslight, may I redirect your attention to the EventPublisherViewModel which contains the following code.

private void ExecuteSendMessage(object parameter)
{
    ((IMultiPageViewModel)this.ParentViewModel).SelectedIndex = 1;
    EventAggregator.Default.Publish<DelegateEvent<string>, string>(this.Message);
}

Instead of invoking the navigation service here, you need to set the SelectedIndex to move from one tab to another. Feel free to ask if you have any further questions. Thanks.