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.