Error: Doesn't have a main class

[makorne]

The plugin `plugin_with_protobuf` doesn't have a main class defined in /home/f/StudioProjects/flutter-plugin-protobuf/android/src/main/java/pro/truongsinh/flutter/example/plugin_with_protobuf/PluginWithProtobufPlugin.java or /home/f/StudioProjects/flutter-plugin-protobuf/android/src/main/kotlin/pro/truongsinh/flutter/example/plugin_with_protobuf/PluginWithProtobufPlugin.kt. This is likely to due to an incorrect `androidPackage: pro.truongsinh.flutter.example.plugin_with_protobuf` or `mainClass` entry in the plugin's pubspec.yaml.
If you are the author of this plugin, fix the `androidPackage` entry or move the main class to any of locations used above. Otherwise, please contact the author of this plugin and consider using a different plugin in the meanwhile. 

Your Flutter application is created using an older version of the Android
embedding. It's being deprecated in favor of Android embedding v2. Follow the
steps at  https://flutter.dev/go/android-project-migration
to migrate your project.