Mainactivity.java file missing #9

In the readme file author said we have to edit Mainactivity.java file but in flutter flutter 1.12.13+hotfix8 there is no such file. When I add it I get the following error

FAILURE: Build failed with an exception.

What went wrong: Execution failed for task ':app:compileDebugJavaWithJavac'.

Compilation failed; see the compiler error output for details.

Try: Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

Get more help at https://help.gradle.org

BUILD FAILED in 8s Gradle task assembleDebug failed with exit code 1