Getting Values From A Custom Property

I've created a button and given it a custom property known as 'customProp'. When I attempt to get the value of the custom property using getCustomProp(), nothing happens. Can someone show me the correct way to include a custom property and retrieve the value?

Someone else did something similar in the following Fiddle. I see that he's defining the button itself, where I am defining the panel and including the button, but I'm not sure why that would make a difference.

Getting Values From A Custom Property