Dart if(A is B) condition not working properly

is condition image

As you can see from the picture, I got a variable called item which is an instance of BookFavorItem.

However, when I use condition if( item is BookFavorItem ) to control the flow, it did not work as expected behavior. (I thought this condition should be true, but it didn't)

What's wrong with me?

-> flutter doctor

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.17.1, on Microsoft Windows [Version 10.0.19041.264], locale zh-CN)
[!] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    X Android license status unknown.
      Try re-installing or updating your Android SDK Manager.
      See https://developer.android.com/studio/#downloads or visit visit
      https://flutter.dev/docs/get-started/install/windows#android-setup for detailed instructions.
[!] Android Studio (version 3.6)
    X Flutter plugin not installed; this adds Flutter specific functionality.
    X Dart plugin not installed; this adds Dart specific functionality.
[√] VS Code (version 1.45.1)
[√] Connected device (1 available)