Ui: Not able to delete Thing/Device from UI, 500 Internal Server Error

What are the steps to reproduce the issue?
Using docker and docker compose
1. git clone https://github.com/mainflux/ui.git
2. cd ui
3. docker-compose -f docker/docker-compose.yml up
4. http://localhost //UI is up // logs are running in the background
5. Register/Sign In with emailId and Password.
6. Go to Things --> Devices --> Click Under Actions "+" Button.
7. New Row is added here we can give only Name rest other fields ie [Type , ID] are NOT editable
8. Give some name say "Device1" and click save ie "Tick" button
9. Now try to delete the Device1 , UI will prompt you for deletion press Submit
10. In UI added row disappears and also it prompts at Right bottom of the screen as "Failed to delete Thing Error:500 Internal server error"

   From Backend logs :
   mainflux-things | {"level":"warn","message":"Method remove_thing for token  eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjE1OTQyNTcxNzIsImlhdCI6MTU5NDIyMTE3MiwiaXNzIjoibWFpbmZsdXguYXV0aG4iLCJzdWIiOiJrbm93bG9raUBnbWFpbC5jb20iLCJ0eXBlIjowfQ.lpsQXA0Yx5L77x0CpVnVnqHGheQItRDMFXj1GjWR-h8 and thing ea3b9e5c-06a0-46cd-944a-2fcce37c8040 took 1.717998ms to complete with error: remove thing failed : remove thing from redis cache error : redis: nil.","ts":"2020-07-08T15:22:31.428751005Z"}

In what environment did you encounter the issue?
DigitalOcean droplet - Ubuntu Docker 5:19.03.1~3 on 18.04