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If `FlutterDartProject` found an `FLTLibraryPath` entry in an iOS
application's `Info.plist`, it assumed that values that were valid
filesystem paths were paths to bundles.  If the attempt to retrieve
the `NSBundle` fails, `FlutterDartProject` ignored the failure and
then would assign `nil` to a C++ `std::string`, resulting in a null
pointer dereference.

Add some failure checks to prevent this.

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…#6043)

The default WindowManager implementation in Android's Presentation is
delegating addView/removeView/updateViewLayout calls to the global
WindowManager.
This can result in a crash when an embedded view is trying to e.g show a
PopupWindow.

This change adds a custom WindowManager that overrides
addView (and removeView/updateViewLayout) and adds the view to the
presentation's view tree.

Note that views might keep a reference to the window manager which
might be an issue when we move a view from one virtual display to
another (due to a resize). For this reason when re-sizing we are not
creating a new window manager for the new presentation, but updating the
window manager's references to be relevant for the new presentation and
re-use it.