The user is getting a warning, not an error message. If it's working otherwise you don't really need to fix anything, just turn off the warning.
This seems to be a bug with the module itself. To quote from the thread I referenced,
If you are seeing this on the site that is not using Ubercart (or Ubercart Webform Checkout Pane) then its a bug with Webform module itself or some other module that is referencing Webform. IMHO you should check with supporters of those modules.
Which is what Steve was alluding to as well. You need to find the conflicting module, and report it to the developer or trouble shoot it.
The thread is closed because they fixed the issue with a patch to Drupal, which should also be in the latest versions, so updating drupal might be a good idea if you haven't already
However if it's not a problem the easiest thing to do is turn of the warning.
This warning appears because of your PHP error reporting settings. Usually, it appears when your variable is not properly set. If you want to find out what the warning means and learn how to deal with it, do a google search for "Notice: Undefined index:"
You can turn off the error messages by going to Configuration > Logging and errors.
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