Your code navigates to:
https://livebuilds.openbravo.com/retail_pi_pgsql/web/org.openbravo.retail.posterminal/?terminal=VBS-1#login
When I tried, the page took nearly 20 seconds to load. I had to wait long before I touch the page. Same applies to your code. Your code should wait enough seconds after calling WebUI.navigateToURL(...) and before calling WebUI.setText(findTestObject(..),...).
Then how you can wait appropriate seconds? There are many post concerning that issue. Start tracing them with this: