Hi,
Sorry about that. Looks like a bug slipped in. This is already fixed in the Gantry CI build and will be released on monday with 5.2.1.
If you want to fix it now please edit the wp-content/plugins/gantry5/src/classes/Gantry/WordPress/Widgets.php line 293 and change this :if (!isset($widget_obj['callback'][0]) || !is_object($widget_obj['callback'][0])) {
to this :if (!is_array($widget_obj['callback']) || !isset($widget_obj['callback'][0]) || !is_object($widget_obj['callback'][0])) {
Sorry about it.
Jakub