It seems that you put the code into the false callback. I ran into this issue too some time ago. There are two callbacks that have a quite similar name what can be mixed up quite easily. onController and onControl. While the OnController manages the CC inputs the onControl serves as the callback for the GUI-Widgets/Elements.
And BTW: be a bit lenient with the tutorial in its current state. There have been many changes in HISE in the last months and not everything seems to be up to date.