On the welcome screen of DataSpell I chose to open a project with Get from Version Control to get a project from GitHub, although in the left pane I didn't choose GitHub, so it created an empty .git repository.
After that, I git cloned elsewhere the git repository or tried to clone it correctly from GitHub but in every occasion I get
Error
Cannot attach project Module name 'project name' already exists.
Trying to load module: /path/to/project name.iml
Where does DataSpell save the imported projects, to detach the empty project that I imported initially?
I managed to open the project following the instructions of Open your project from disk section of the DataSpell site.
For avoiding further trouble, I opened the project in a new window. After the configuration in step 2 Reopen projects on startup --> Ask, Dataspell will ask how to open the project.