Repeated Popup: Xcode wants to access key “com.apple.dt.XcodeDeviceMonitor” in your keychain

Posting this solution for Xcode 8 because no one else has:

1. Open Keychain Access.
2. Search for XcodeDeviceMonitor.
3. Drag the item to the System Keychain on left.
4. Enter admin password.

That finally fixes it.