Flutter - TextField focus changes TabBar selected index

I have a Scaffold, within it I have a TabBar , TabBarView and a TextField .

The TabBar has 3 tabs (e.g. tabs A, B and C), the TabBarView has 3 views and this TextField is at the last tab (tab C).

Everything is working, but whenever I put the focus on the TextField to type something, the TabBar is changed from tab C to tab A. Very annoying. This should not happen. The TabBarView remains unchanged.

I created the controller in the initState . like this:

@override
void initState() {
  super.initState();
  widget._tabBarController =
      new TabController(length: 3, vsync: this);
}

Any idea why it happens?