Whilst I am very much a Velo newbie myself - I have found that, in general, the error message “Cannot read property “someProperty” of undefined” seems to be a Javascript error which gets raised by your browser whenever it parses some script that attempts to access (or modify) a page element (object) which “has not yet been created” in the browser.
Maybe you could try moving your code and placing it inside the Web Page’s “On Ready” event handler. This way, you can know that all the page elements have been created and registered in the brower’s DOM - before you attempt to interact with them.??