I'm the author of a WordPress plugin and had a customer that was
having trouble with Gantry.  Basically, adding widgets via my plugin
was causing errors to be thrown by Gantry.

Warning: Invalid argument supplied for foreach() in /home/laundro/
public_html/wp-content/plugins/gantry/core/renderers/
gantrywidgetsrenderer.class.php on line 154

After digging through the Gantry code for a while, this is what I've
found:

From what I can tell, Gantry augments the WordPress widgets with a
"widget_map" (not sure exactly its purpose, but I don't think it's a
core WordPress feature).  Then Gantry adds a filter to the widgets,
and it assumes this widget_map exists, attempting to access it in an
array.  I'm guessing the widget_map doesn't exist because this is a
widget area added by a plugin rather than Gantry.  However, the filter
is applied to all widgets and doesn't check to see if the widget_map
exists.  Since it doesn't, it throws an error.  Since the filter
throws an error, it returns null, and the parameters it filters get
cleared, so this actually causes 2 errors, and the second prevents the
widget from being displayed.

The (potential) solution:

Add a check to the Gantry function to ignore widgets without the
widget_map parameter:

if( !isset( $params[0]['widget_map'] )) return $params;

This would be placed in plugins/gantry/core/renderers/
gantrywidgetsrenderer.class.php, around line 148, so you end up with
this as the filterWidget() function:

function filterWidget($params) {

        if( !isset( $params[0]['widget_map'] )) return $params;

        global $gantry;

        $widget_id = $params[0]['widget_id'];
        $layout = $params[0]['layout'];

        // find the widget and its position
        foreach ($params[0]['widget_map'] as $pos => $position_info){
            $position_widgets = $position_info['widgets'];
            if (!array_key_exists($widget_id,$position_widgets))
continue;
            $params[0]['position'] = $pos;
            $params[0]['end'] = end(array_keys($position_widgets));
            $params[0]['start'] =
reset(array_keys($position_widgets));
            break;
        }

        $params = $gantry->renderLayout('widget_'.$layout, $params);

        return $params;
    }

The only alternative I could find would be to temporarily remove the
filter in any other plugin in order to avoid the error:

        if(function_exists('dynamic_sidebar')){

                //Gantry-specific
                remove_filter('dynamic_sidebar_params',
array('GantryWidgetsRenderer', 'filterWidget'));

                ob_start();
                echo '<ul id="mysidebar">';
                dynamic_sidebar($name);
                echo '</ul>';

                //Gantry-specific
                add_filter('dynamic_sidebar_params', array('GantryWidgetsRenderer',
'filterWidget'));

                return ob_get_clean();
        }

Of course that's just a hack and clearly not optimal.  A filter
shouldn't throw errors because a non-standard field is missing when
calling a standard WordPress function.

Hope you'll either add that fix or a comparable one to your next
release!  It doesn't seem like something that can be solved gracefully
from outside your framework.

Thanks!

Chris