Dart APIunself_componentUnSliverArtBaraction action property Null safety Widget? action final Implementation final Widget? action; unself_component unself_component UnSliverArtBar action property UnSliverArtBar class Constructors UnSliverArtBar Properties action background description emoji hashCode isImmersive key leading name runtimeType trailing Methods createElement createState debugDescribeChildren debugFillProperties noSuchMethod toDiagnosticsNode toString toStringDeep toStringShallow toStringShort Operators operator ==