Dart APIunself_componentUnselfNavigationBarpreviousPageTitle previousPageTitle property Null safety String? previousPageTitle final Implementation final String? previousPageTitle; unself_component unself_component UnselfNavigationBar previousPageTitle property UnselfNavigationBar class Constructors UnselfNavigationBar Properties automaticallyImplyLeading automaticallyImplyMiddle backgroundColor border brightness hashCode heroTag key leading middle padding preferredSize previousPageTitle runtimeType trailing transitionBetweenRoutes Methods createElement createState debugDescribeChildren debugFillProperties noSuchMethod shouldFullyObstruct toDiagnosticsNode toString toStringDeep toStringShallow toStringShort Operators operator ==