never mind guys, i was able to resolve the issue as follow.
I used to use AppDelegate inside my app to launch FirebaseApp.configure() but when using the SwiftUI Scene modifier .backgroundTask, it seems there is a run on the view model (where I initialize Firestore and FirebaseStorage).
I noticed that AppDelegate takes longer time than the launching of the app or basically the scene, therefore the app started trying to access view model before AppDelegate completed its work. Accordingly the FirebaseApp.configure() not yet initialized.
So I moved the FirebaseApp.configure() from AppDelegate to the view model init() before the initialization of Firestore and FirebaseStorage inside the init(). and it Worked!!!!!
I don't know if this way is appropriate or is there a better way to implement.
If any of you guys can comment on which way is better (keeping the FirebaseApp.configure() in the AppDelegate or just move it to the view model)?