If you create a group discussion and put a column in the grade center, then delete the group discussion, the following error will occur.

To fix the error please follow the steps listed below.

1. Open up the course in Firefox.
2. Click on Full Grade Center.
3. Click on Manage (as shown in the image above) and click on Column Organization.
4. Once in the Column Organization area, under "Not in a Grading Period" - move a column. It doesn't matter what column or where you move, just move a column and click Submit.

You will be taken back to the Full Grade Center and the error will be gone.