Codigo original:
onChanged: (date) {
setState(() {
_selectedDate = date;
});
},
solución:
onChanged: (date) {
(context as Element).markNeedsBuild();
_selectedDate = date;
},
Documento de referencia:
7.7 Detalles del cuadro de diálogo: https://book.flutterchina.club/chapter7/dailog.html#_7-7-1