For online text assignments, the Export to portfolio button on the grading page does nothing.

To reproduce:

1. As an admin, go to Site administration > Advance features > Enable portfolios
2. Go to Site administration > Plugins > Manage portfolios and enable one or more portfolio plugins.
3. As a teacher, enter a course and create an assignment with the online text option enabled.
4. As a student, open the assignment, enter some text, and save.
5. As a teacher, open the assignment and select the 'Grade' button.
6. Click the 'Export to portfolio' button below the submitted text - nothing at all happens.

If there is  an uploaded file, the 'Export to portfolio' link below the file submission works.

In addition, if you go back to the assignment page and select 'View all submissions', you can export the text to a portfolio from there.

Exporting online text from the grading page does work as expected in an earlier version (3.1.6).

[Also, 'Export to portfolio' is a button for online text, and a link for submitted files. That's rather odd.]