warning: Invalid argument supplied for foreach() in...

there is empty variable $conflict->{$field} when you use more then one resource conflict contents, or maybe it depends on my module configuration

patch:

        foreach($fields as $field) {
          foreach($node->{$field} as $instance) {
            foreach ($overlap_nodes as $conflict) {
			  
			  // my code :) don't ask me why ;))
			  if (empty($conflict->{$field})) { continue; }
			  
              foreach($conflict->{$field} as $conflict_instance) {
                if ($instance['nid'] == $conflict_instance['nid']) {
                  $booked_resource = node_load($instance['nid']);
                  form_set_error('Resource Conflict', 
                    t('There is a resource conflict. "%resource" is currently booked. Please choose a different time or a different resource.',
                      array('%resource' => $booked_resource->title)));
                }
              }
            }
          }
        }