High Sierra. Disabled SIP, copied XtraFinderInjector.osax to /System/Library/ScriptingAdditions. Copied XtraFinder.app to directory /Applications, re-enabled SIP.
Run XtraFinder, get this:
could not find file /system/library/scriptingadditions/xtrafinderinjector.osax
Check directory, it is there. Disable SIP just in case, same error message.
Uninstaller works as intended.