[exception] groovy connector : script is null

Hello,

I'm using Bonita BPM Community Edition v.7.2.1

Development > Manage groovy scripts
I created groovy script named 'GetId'.
and the contents are as follows.

public def static hello()
{
    return 'hello'; 
}

And I made a service task and it has a connector. I used Groovy 2.4 connector.
and its scripts are just only GetId.hello();

When I test at the Edit expression window using 'Evaluate' button, it works well. it shows 'hello' on pop-up.
BUT.. when I test using 'Test' button, it prints as follows.

java.lang.reflect.InvocationTargetException
org.bonitasoft.engine.bpm.connector.ConnectorExecutionException: USERNAME=install | org.bonitasoft.engine.core.connector.exception.SConnectorException: org.bonitasoft.engine.connector.exception.SConnectorException: java.util.concurrent.ExecutionException: org.bonitasoft.engine.connector.exception.SConnectorValidationException: org.bonitasoft.engine.connector.ConnectorValidationException: Error validating connector org.bonitasoft.connectors.scripting.GroovyScriptConnector:
The script is null

Could you explain why it happens??

Thanks a lot!