This problem has appeared because we no longer require the "Introductory Message" (intro_text) field on page 1 of the wizard. The Widget 'about' field default value is populated from that field - and now it can be empty.
To fix this we need to modify Widget.php to set all fields to required = false in buiildQuickForm. Then add a formRule function which enforces the existing required field rules IF is_active is TRUE.
Also please fix this NOTICE: which occurs when intro_text is empty...
Notice: Undefined index: intro_text in xxxxxxxxx\administrator\components\com_civicrm\civicrm\CRM\Contribute\Form\Contribution\Main.php on line 61
----- Original post from Elin Waring ----
When you get to the widget page and leave the box unchecked and attempt to go to the next page, it gives you a warning that says About field is required.