Flexi table: cannot select only one column with mandatory columns

How to reproduce

• Open "Courses"
• Click on the settings wheel (right side of the table) 
• Click on "Delete selection"
• Select only one column
• Click on "Save"
• => NOK Table remains unchanged
• Table should show only one column

• Open "Groups"
• Click on the settings wheel (right side of the table)
• Click on "Delete selection"
• Click on "Save"
• => NOK Table remains unchanged
• Table should show only the mandatory three columns