I found a work-around (for Windows).
Close all programs.
Open the “Run” command prompt by pressing the Windows logo key and R on the keyboard simultaneously. The Windows key is located between the keys Ctrl and alt.
(To open the command prompt on Windows XP select ‘Run’ from the Start menu.)
In the command line of the ‘Run’ window, enter this path: %appdata%/Steinberg
Copy the file named “Midi Devices.bin” from the previous version of Cubase.
Go back and open the folder that is named after the current version of Cubase and rename the Midi Devices.bin to something logical.
Paste in the original Midi Devices.bin file.
Open Cubase 9 and your Midi Device names should be there.