The software queries for the miners you have added and if an IP does not respond then the miner will be added to list of in-active miners.
I suspect that after you physically removed the miner another device got the same ip, which is not a miner so the app crashed because it tries to communicate with the miner's API.

Please check if this is the case and make sure that no other device has the IP your miner had.
Normally you should first delete the device from the application and then unplug it.

If the problem continues you can delete the database and add the miners again.

Let me know if that helped