The project concept in the old Developer Rig was realized in the browser’s local storage. In the new Developer Rig, it is realized as JSON file in the project folder. An extension manifest, as provided in the response to the API the “refresh manifest” button invokes, is not a Developer rig project. Your best option with the new Developer Rig is to create a new project, selecting your extension from the “Select Existing or Create New Extension” drop-down list and entering your desired version in the “Extension Version” input.