Prevents a white screen when a viewer is set but not enabled.

If a user selects a viewer and then disables that module, but does
not change any solution packs viewers settings this can occur.

includes/solution_packs.inc

@@ -745,9 +745,11 @@ function islandora_get_viewer($params = NULL, $variable_id = NULL, $fedora_objec
     $viewer_id = islandora_get_viewer_id($variable_id);
     if ($viewer_id AND $params !== NULL) {
       $callback = islandora_get_viewer_callback($viewer_id);
+      if (function_exists($callback)) {
         return $callback($params, $fedora_object);
       }
     }
+  }
   return FALSE;
 }