Due to a bug in the KUKA robot component FlexLink Design Tool crashes when an RI20 robot cell is deleted from the 3D-world.

Workaround

1. Select the robot

2. Unlock the robot in the properties panel

3. Disconnect the robot from the cell (drag-and-drop outside the cell)

4. Select the cell

5. Press Delete 

6. Select the robot

7. Press Delete