eSewa ePay integration in flutter with webview supports flutter 2.0 no need to downgrade your flutter SDK (flutter 2.0) for implementing esewa payment integrations at the moment but in future esewa package library will also be migrated to null safety , until then you can work with esewa with this approch & can work in future too cause webview approach is a bit convinient and simple IMO ..
lets keep learning and sharing !!