hook - Hooking Android method with Frida but .implementation() not

I am learning Frida, and trying to hook a method checkFlag and trying to modify the boolean flag from true to false. Below is my demo app code In normal scenario, the Toast with Flag is True is

💉 Quick Start with Frida to Reverse-Engineer Any iOS Application

Android Reverse Engineering Guide · OWASP MSTG Summit Edition

Instrumenting Native Android Functions using Frida

Android Penetration Testing: Frida - Hacking Articles

Captain Hook - How (not) to look for vulnerabilities in Java

Frida basics - Frida HandBook

Two Attack Scenarios that Will Defeat Your DIY iOS App Protection

Invoking Uncalled Native Function on Android APK Using Frida, by Syahrul Akbar R

Frida Hooking Native Functions. Are you ready for Part 3?! This is the…, by Ashlyn Lau

Hooked method contents ignored · Issue #371 · frida/frida · GitHub