The file build.gradle in the app folder shows an error(shown in the attached image). But when I run the app it works just fine. Should I be concerned about it?

1 Answer
No You don't have to worry about it.
If you open your Android project in Android studio then this errors will gone.
To open Android module in Android studio.
Right click on your project => Flutter => Select Open Android module in Android Studio.
Update
You can enable the "Enable code completion, navigation, etc. for Java/Kotlin." from preferences.
Prefrences/Settings => Languages and Frameworks => Flutter => Below a Experiments tab.
This feature is still in experimental.