The code in screenShield.js does:

            this._consoleKitProxy = new ConsoleKit.ConsoleKitManager();
            this._consoleKitProxy.ckSession.connectSignal('Lock', Lang.bind(this, function() { this.lock(false); }));
            this._consoleKitProxy.ckSession.connectSignal('Unlock', Lang.bind(this, function() { this.unlock(); }));

But ConsoleKitManager.ckSession is asynchronously initialized so it doesn't exist yet at this point. The solution might to be add gjs signals to the ConsoleKitManager object for Lock/Unlock.