See https://github.com/flutter/flutter/pull/103719

This reverts commit 5fd259be.

Adds Navigator.maybeOf to replace calling Navigator.of(context, nullOk: true), and removes the nullOk parameter. Also changes Navigator.of to return a non-nullable value, and removes many (120!) instances of the ! operator, reducing the possible places where a null dereference could occur.

This reverts commit 7ba775a9.