I did a small test now and connected latest version of Hue Essential with my local instance. Here it worked without issues 
The deCONZ instance was shown, I selected it and in the Browser via Phoscon App → Menu → Gateway → Advanced Settings, clicked on connect 3rd party apps. Then Hue Essentials was connected.
Looking closer at the error message above I don’t think this has anything to do with authentication but that Hue Essentials likely expects the on property to only contain true or false but not null.
Not sure why there is a null in first place (same for bri), will check with the devs.