Advice needed wrt bindables Aurelia 2 beta 18

In a Auralia 1 application I have a component/class that has this piece of code in it:

I’m trying to port this class to Aurelia 2 beta 18 and have tried the following:

When attached() gets called the following code is executed:

In Aurealia 1 this works just fine. In my version for Aurelia 2 the last statement throws an exception because this.navitems[0] apparently is undefined (or maybe it was null. Can’t remember).

Can anyone point me in the right direction so I can get this to work?