On October 15th, I had this issue.

1. I went to Tools > Device Manager and noticed my attached device was not showing.
2. From there I went to my attached device, went to the notifications shade selecting the Android System USB notification.
3. I then turned on the "File Transfer / Android Auto" option under the "USE USB FOR" section of the USB Preferences setting menu.

This fixed the "loading devices" issue for me. I am only posting my answer because while I am sure the ADB kill option may work, my solution kept me from rebooting anything. If you are Linux fan boi like me then you know its all about up time. Also my answer had not been posted yet and so I figured it provide someone seeking a solution more context in their journey.