iOS FlutterViewController splashScreenView made nullable

The FlutterViewController property splashScreenView has been changed from nonnull to nullable.

Old declaration of splashScreenView:

@property(strong, nonatomic) UIView* splashScreenView;

New declaration of splashScreenView:

@property(strong, nonatomic, nullable) UIView* splashScreenView;

Prior to this change, on iOS the splashScreenView property returned nil when no splash screen view was set, and setting the property to nil removed the splash screen view. However, the splashScreenView API was incorrectly marked nonnull. This property is most often used when transitioning to Flutter views in iOS add-to-app scenarios.

While it was possible in Objective-C to work around the incorrect nonnull annotation by setting splashScreenView to a nil UIView, in Swift this caused a compilation error:

error build: Value of optional type 'UIView?' must be unwrapped to a value of type 'UIView'

PR #34743 updates the property attribute to nullable. It can return nil and can be set to nil to remove the view in both Objective-C and Swift.

If splashScreenView is stored in a UIView variable in Swift, update to an optional type UIView?.

Code before migration:

  var splashScreenView = UIView()
  var flutterEngine = FlutterEngine(name: "my flutter engine")
  let flutterViewController = FlutterViewController(engine: flutterEngine, nibName: nil, bundle: nil)
  splashScreenView = flutterViewController.splashScreenView // compilation error: Value of optional type 'UIView?' must be unwrapped to a value of type 'UIView'

Code after migration:

  var splashScreenView : UIView? = UIView()
  var flutterEngine = FlutterEngine(name: "my flutter engine")
  let flutterViewController = FlutterViewController(engine: flutterEngine, nibName: nil, bundle: nil)
  let splashScreenView = flutterViewController.splashScreenView // compiles successfully
  if let splashScreenView = splashScreenView {
  }

In stable release: 3.7

Relevant PR:

• Make splashScreenView of FlutterViewController nullable

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