Fixed onWillPop assert error

[yu1ro]

fix #308

[ayoubm]

@BilalShahid13 can you please merge this PR? It's needed for the willPopScope to work properly.

Thank you!

[pishguy]

@yu1ro +1

[htunlin-kitesense]

Hello, can help merge this PR? willPopScope not working properly in PersistentTabView.custom.

[danielcabral-code]

Hey, would be nice if someone could merge this please. As @htunlin-kitesense mentioned, PersistentTabView.custom is currently impossible to use.

[AndreSait]

In flutter 3.7.8, the onWillPop creates some casting problems, which can be fixed by changing it to this:

onWillPop: !widget.handleAndroidBackButtonPress && widget.onWillPop != null ? () async { final result = await widget.onWillPop!(_contextList[_controller!.index]); return Future.value(result); }

[pupubird]

Please help to merge this PR :c

[pupubird]

For anyone ever come to this issue, here's the V2 that's actively maintained https://pub.dev/packages/persistent_bottom_nav_bar_v2