This is caused by the introduction of https://github.com/Graylog2/graylog2-server/pull/5853. Actually it is reflected in our maven archetype you probably have used to generate the boilerplate for your plugin. Maybe you did that before this PR got merged?
Oh. The Sample Plugin is unfortunately not actively maintained anymore, I am adding a notice to the README indicating that it should not be used as a template. We are currently updating the docs to correct a number of mistakes in there and streamline the process of creating new plugins. Thanks for your input!