Sorry to know that you are having issues, it seems like the error doesn't related to Upfront Builder. You have declared the following function _check_isactive_widgets() twice, most probably in your functions.php file of your Colormag theme, or some other place.
You'll have to make use of a FTP client, in order to check your functions.php file. If you aren't sure about where you have made the edits, you can revert back to the old copy of your functions.php file.
If you still have issues, and if you want to get your site up, renaming the theme folder, located in your /wp-content/themes/ will give you access to your dashboard, ie changing colormag to test-colormag, will switch your theme to default WordPress theme. You'll still have to go through your theme files, to figure out where the error is, and rename the folder back from test-colormag to colormag in order to activate your theme.
If you aren't sure about how to make these changes, or if you still have issues. You can send us your credentials, so that we could give a closer look.