Bartender 3 has just been released, it's fully compatible with macOS High Sierra (and macOS Sierra) and does not need the below steps performed.
We recommend all users should update to Bartender 3 for FREE, if you have a Bartender 2 license.
Bartender 2 cannot control system items in macOS High Sierra, you can update to Bartender 3 for FREE if you have a Bartender 2 license.
NOTE: DO NOT APPLY THE SYSTEM COMPONENT ON macOS High Sierra
As of macOS High Sierra Apple have expanded SIP (System Integrity Protection) and made it impossible to control System Items as we have been doing. Applying the System Component will currently cause a crash in System Items
If you are on macOS Sierra or older and wish to use Bartender 2 and install the system component instructions can be found here