By any chance are you using the text field as part of a data entry form or similar and then using submit at which point the error occurs? You might want to try this:
1) Click on the text field
2) Check the Properties sidebar, Data section 'required' is unticked
If that doesn't solve it, the way I did on my project (done using JBP 2.4.1 as I haven't moved to 2.6 yet) was to check for the text field for null and then set it to be a value if null and set in the sessionbean. Assuming you are using a submit button, then put that check in the button_action() method.
If you are setting data into the textfield, make sure it is in the prerender() method.
Also, are you using a dataset? Sometimes if you have drag n dropped a database table onto the page and deleted it, it will remain there until you remove it from the code page or even the sessionbean.java code - you do get the error if you have used a parameter query and there is no query there for it run against (i.e. you have a '?' in your sql statement). In other words, remove all traces of any unused data providers.