Hi Nguyen,
Somehow I missed your response, sorry.
As Dennis already mentioned you have to listen to attachRequestCompleted event then only you can get the IconTabBar items list. Basically it works like this, on load of page you are loading the json file. Application will hit onInit first where you're JSON model file is not loaded with data yet. Then later it hits the attachRequestCompleted where you have the data available in your model. You can do the manipulations there.
One more thing is your code works fine with out attachRequestedCompleted as well if you add this attribute in your UI5 bootstrap - data-sap-ui-xx-bindingSyntax="complex". But, you cannot fetch number of items as I explained the issue root cause already. I have done few changes to your code. All the changes are done in index.html - Plunker
Regards,
Sai Vellanki.