If you're developing an Effect, Title, Transition or Generator, you can't really because the Motion Project/FCP Template relies on specific information about those "states" and it is assumed you will eventually have a finished published template. An Effect has an "Effect Source"; Titles have a "Title Background"; Transitions have *two* "background sources"; Generators are simply compositions directly published to Final Cut as a template.
If you're just "messing around", sometimes it helps to start out with a plain Motion Project - also known as a "Composition".
At any given time, you can go to File > Convert Project To > ... and choose what type of template or project you want to convert to -- to save.

To answer your question now - with the project you're working on — Cancel out of that dialog. Convert Project To > Composition and Save it wherever you want to. Later you can use what you've been working on and apply it to any of the other template types to be published to Final Cut.