How to run the akka grpc java example

Ok, following the prerequisites and instructions to use Java 8 the project creation works.

sbt new akka/akka-grpc-quickstart-java.g8

Then continuing with the guide

./gradlew compileJava works as well.

but then ./gradlew --info runServer fails with the folling errors/issues:

Task ':runServer' is not up-to-date because:
Task has not declared any outputs.

and

Error: Main method not found in class com.example.helloworld.GreeterServer, please define the main method as:
   public static void main(String[] args)

ORIGINAL POST:

I am having trouble getting the Akka-GRPC example to run. I tried following this guide here but when I download the sample project it seems incomplete.

Thats the tree command output:

.
├── build.sbt
├── project
│   ├── build.properties
│   └── plugins.sbt
├── sbt
├── sbt.bat
└── sbt-dist
    ├── bin
    │   ├── sbt
    │   ├── sbt.bat
    │   ├── sbt-launch.jar
    │   └── sbt-launch-lib.bash
    └── conf
        ├── sbtconfig.txt
        └── sbtopts

4 directories, 11 files

There are no source files or gradlew oder pom.xml in the project structure.

Next I cloned the repository of that example from GitHub and followed the instructions in readme, mainly the command sbt new akka/akka-grpc-quickstart-java.g8.

$ sbt new akka/akka-grpc-quickstart-java.g8
[info] Loading settings for project akka-grpc-quickstart-java-g8-build from paradox.sbt,giter8.sbt ...
[info] Loading project definition from /home/johannes/IdeaProjects/akka-grpc-quickstart-java.g8/project
error: error while loading String, class file '/modules/java.base/java/lang/String.class' is broken
(class java.lang.NullPointerException/null)
[error] java.io.IOError: java.lang.RuntimeException: /packages cannot be represented as URI
...//had to remove stacktrace was too long for this post
[error]     at xsbt.boot.Boot.main(Boot.scala)
Project loading failed: (r)etry, (q)uit, (l)ast, or (i)gnore? [error] java.io.IOError: java.lang.RuntimeException: /packages cannot be represented as URI
[error] Use 'last' for the full log.

then I went into the project/sources folder ./src/main/g8/ where the source files are located and tried to follow the instructions from the guide from in there. That also did not work because of some properties errors:

Before that I had to set another gradle version (6.1.1, was 4.5.1) in ./gradle/wrapper/gradle-wrapper.properties to make it compatible with Java 13.

$ ./gradlew compileJava

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring root project 'g8'.
> Could not resolve all artifacts for configuration ':classpath'.
   > Could not find gradle.plugin.com.lightbend.akka.grpc:akka-grpc-gradle-plugin:$akka_grpc_version$.
     Searched in the following locations:
       - file:/home/johannes/.m2/repository/gradle/plugin/com/lightbend/akka/grpc/akka-grpc-gradle-plugin/$akka_grpc_version$/akka-grpc-gradle-plugin-$akka_grpc_version$.pom
       - https://plugins.gradle.org/m2/gradle/plugin/com/lightbend/akka/grpc/akka-grpc-gradle-plugin/$akka_grpc_version$/akka-grpc-gradle-plugin-$akka_grpc_version$.pom
     Required by:
         project :

* 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 4s

Trying to at least temporarily get this to work i hardcoded the properties into the build.gradle file:

     //classpath 'gradle.plugin.com.lightbend.akka.grpc:akka-grpc-gradle-plugin:$akka_grpc_version$'
     classpath 'gradle.plugin.com.lightbend.akka.grpc:akka-grpc-gradle-plugin:0.7.3'

But that also only got me so far:

$ ./gradlew compileJava

FAILURE: Build failed with an exception.

* Where:
Build file '/home/johannes/IdeaProjects/akka-grpc-quickstart-java.g8/src/main/g8/build.gradle' line: 20

* What went wrong:
A problem occurred evaluating root project 'g8'.
> Failed to apply plugin [id 'com.google.protobuf']
   > Could not create an instance of type com.google.protobuf.gradle.ProtobufSourceDirectorySet.
      > 'void org.gradle.api.internal.file.DefaultSourceDirectorySet.<init>(java.lang.String, java.lang.String, org.gradle.api.internal.file.FileResolver, org.gradle.api.internal.file.collections.DirectoryFileTreeFactory)'

* 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 1s

Again could not include whole stacktrace because of character limit.

What am I missing? Which approach should I pursue further?