don't crash if anchorEl doesn't exist

public/src/modules/navigator.js

@@ -143,10 +143,12 @@ define('navigator', ['forum/pagination', 'components'], function(pagination, com
 				threshold = 0;
 			} else {
 				var anchorEl = components.get('post/anchor', index - 1);
+				if (anchorEl.length) {
 					var anchorRect = anchorEl.get(0).getBoundingClientRect();
 					threshold = anchorRect.top;
 				}
 			}
+		}
 
 		if (typeof navigator.callback === 'function') {
 			navigator.callback(index, count, threshold);