Flutter android build.gradle unable to resolve class GradleException

In my flutter app, flutter build apk succeeds without warning but android.app.build.gradle has an error in this code:

def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}

The error is

unable to resolve class GradleException

whereis flutter returns "/home/mark/dev/flutter/bin/flutter".

flutter doctor -v returns Flutter version 3.3.1 on channel stable at /home/mark/dev/flutter` and has no errors.

my local.properties file contains flutter.sdk=/home/mark/dev/flutter

How can I resolve this error?