Dart APIunself_componentUnselfSliverNavigationBarpreviousPageTitle previousPageTitle property Null safety String? previousPageTitle final Implementation final String? previousPageTitle; unself_component unself_component UnselfSliverNavigationBar previousPageTitle property UnselfSliverNavigationBar class Constructors UnselfSliverNavigationBar Properties automaticallyImplyLeading automaticallyImplyTitle backgroundColor border brightness hashCode heroTag key largeTitle leading middle opaque padding previousPageTitle runtimeType stretch trailing transitionBetweenRoutes Methods createElement createState debugDescribeChildren debugFillProperties noSuchMethod toDiagnosticsNode toString toStringDeep toStringShallow toStringShort Operators operator ==