I have been using the v23.0.1 support library until now with no problems. Now when I switch to the new v23.1.0 library I am getting a null
It appears attaching the header view to the navigation drawer using xml is currently broken. The solution is to inflate and attach the view manually.
activity layout
app:menu="@menu/drawer_items" />
Then in your code inflate and attach the header by doing the following.
NavigationView navigationView = (NavigationView) findViewById(R.id.navigation_view);
View drawerHeader = navigationView.inflateHeaderView(R.layout.drawer_header);
TextView username = (TextView) drawerHeader.findViewById(R.id.username_textView);