Patrick 
Has successfully completed the online course IntroductionHi Armin,
thanks for your answer! I already imported the standard content on March 6th. The OpenUI5 version is 1.71 and that’s what my screenshots above show as well. I applied the manual changes in the widget script (e.g. I even successfully added the icon there and the TabContainerItem is already referenced in the TabContainer script).
I think the thing is that I still haven’t understood the interaction between the binding of the collection (as a data source) and the model of the TabContainer. That doesn’t seem the same, does it?
Case 1:
If I only use the binding (push a struct into a collection), then a TabItem is created, but I get the problem with the double ID as soon as I want to create a new TabItem (new struct) after removing one – so, as already described above.
Case 2:
If I just use the widget methods (add and remove) instead, then there are no ID issues, but there is also no binding to my data source: The TabItems exist, but the collection remains empty.
Case 3:
Do both have to be implemented (methods of the TabContainer and methods for the collection as data source)? If so, how is that intended? So when adding a new TabItem, do I have to execute the add function of the widget and also push the struct into the collection (in the same way when removing, first get the remove function of the widget and then the struct from the collection)? There is an example in the screenshot!
A little tutorial or a Simplifier sample project would be very helpful! is this possible for you?