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