Runtime error when switching languages

[chris-steele]

If one or more pageNav components exist in the language that is currently viewed, then when switching to another language the following error occurs:

Uncaught (in promise) TypeError: currentPage is null
    getNextPage PageNavModel.js:124
    getButtonTypeModels PageNavModel.js:81
    getNavigationData PageNavModel.js:31
    setupItemsModel PageNavModel.js:19
    Backbone 4
    updateLocation router.js:388
    onLanguageChange router.js:585
    Backbone 5
    set lockingModel.js:70
    setLanguage languagePickerModel.js:42
    onDoChangeLanguage languagePickerDrawerView.js:58

A consequence of this change and probably just needs a guard like this.