the SetState doesn't refresh UI

[er2guotou]

Hi, I run this original code without any modification and find the drop-down button doesn't work as expected.
my env as follows,
[✓] Flutter (Channel beta, v1.0.0, on Mac OS X 10.14 18A391, locale en-SG)
[!] Android toolchain - develop for Android devices (Android SDK 28.0.3)
! Some Android licenses not accepted. To resolve this, run: flutter doctor
--android-licenses
[✓] iOS toolchain - develop for iOS devices (Xcode 10.1)
[✓] Android Studio (version 3.2)
[!] VS Code (version 1.30.1)
[✓] Connected device (1 available)

Could you help me with it?
Many thanks.