Flutter で Android戻るボタンを使うとアプリが落ちてしまう対処法

これでいけるよ。

onWillPop.dart

  Future<bool> _onWillPop() async {
    final isFirstRouteInCurrentTab =
        !await _navigatorKeys[_currentTab].currentState.maybePop();
    if (isFirstRouteInCurrentTab) {
      if (_currentTab != TabItem.home) {
        _onSelect(TabItem.home);
        return false;
      }
    }
    return isFirstRouteInCurrentTab;
  }

🥳