SingletonFlutterWindow window is a global variable that represents the only window of a single-view Flutter app. Since flutter/engine#42991, this variable is a normal FlutterView with ID of _implicitViewId!, which will crash the app if the implicit view is disabled. We must address this issue by the end of phase 2 when the runners are allowed to disable the implicit view.

Part of:

• Allow apps to disable the implicit view #144633

Goal:

• Using window in whatever ways should not trigger errors when the implicit view is disabled.
• Instead of mapping window to an arbitrary existing view, which should provide expected behaviors: the window corresponds to the implicit view, such as FlutterEngine.viewController.

Conclusion:

• Using window when the implicit view is disabled should be no-ops.
• All methods that accept a FlutterView should check whether the view refers to a disabled implicit view.