When adding an interaction to a constituent record you may have clicked in the Participants section by accident, but clicked out since you don't need to add a participant. 
  1. To the left of the blank participant line, click the gray box to highlight the row
  2. Press Delete on your keyboard
  3. Click Yes on the pop-up window that asks "Are you sure you want to delete the selected row?"
  4. Save the interaction when ready