Just wanted to post this here, because it happened to me.
If your following along the tutorial at the end and are wondering why your UI doesnt show up, after making the custom property.
This is likely because the function needed to actually tell blender to use/register the UI to the rig is missing (at least it was for me). You can simply add it as I have done it below.
TLDR: Add this at the end of the script file in text editor:
(make sure the 'if' doesnt have any spaces before it after pasting)
if __name__ == "__main"__:
register()
