NullPointerException when trying to do locking on multiple drawers.

I've got dual navigation drawers on my View. I need to be able to lock one closed when the other is open. My approach is to try and extend the DrawerToggler to support multiple drawers, but when I add the conditional checks

        if (drawerView == _rightDrawerView)
        {
            _drawerLayout.SetDrawerLockMode(DrawerLayout.LockModeUnlocked, _leftDrawerView);
        }
        else
        {
            _drawerLayout.SetDrawerLockMode(DrawerLayout.LockModeUnlocked, _rightDrawerView);
        }

The app quits without a break, and throws a NullPointerException

Here's a complete Gist with the included exception.