Fixed NPE in reflink previews

boards/static/js/refpopup.js

@@ -27,7 +27,7 b' function showPostPreview(e) {'
     //ref id
     var pNum = $(this).text().match(/\d+/);
 
-    if (pNum.length == 0) {
+    if (pNum == null || pNum.length == 0) {
         return;
     }