I just reinstalled the version from GitHub on macOS 10.13.3 (High Sierra), without any problem.
It looks like you fell victim to Apple's increasingly tighter security measures  that intend to protect users from applications not downloaded from the Mac App Store and/or that do not carry a signature from a developer who is a member of Apple's developer program.
Please make sure that in Preferences -> Security & Privacy, you allow apps both from the App Store and from identified developers. Now, do a right-click (ctrl-click) on the Toolbox application and select Open. You'll see a dialog containing a warning that the application is from an unidentified developer, but nevertheless giving you the option to open the application. Click on "Open", and the Toolbox should start normally.
Note that this procedure is necessary only the first time you open the app, subsequently you can open it just by double clicking.
Hope this helps,