digest: only try to show feed loading indicator when feed is actually present on screen

digest.js

@@ -253,12 +253,14 @@ function viewfeed(feed_id, offset, replace, no_effects, no_indicator, callback)
 
 		console.log(query);
 
+		if ($("F-" + feed_id)) {
 			var img = $("F-" + feed_id).getElementsByTagName("IMG")[0];
 
 			if (img && !no_indicator) {
 				img.setAttribute("orig_src", img.src);
 				img.src = 'images/indicator_tiny.gif';
 			}
+		}
 
 		new Ajax.Request("backend.php",	{
 			parameters: query,