Index: modules/block/block.js
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.js,v
retrieving revision 1.9
diff -u -p -r1.9 block.js
--- modules/block/block.js	21 May 2009 21:12:22 -0000	1.9
+++ modules/block/block.js	1 Jun 2009 16:10:19 -0000
@@ -11,11 +11,18 @@ Drupal.behaviors.blockDrag = {
   attach: function (context, settings) {
     var table = $('table#blocks');
     var tableDrag = Drupal.tableDrag.blocks; // Get the blocks tableDrag object.
-
+    if (!tableDrag) {
+      // If the block page is loaded using ajax and then removed, tableDrag
+      // and tableDrag.row may not be defined.
+      return;
+    }
+    
     // Add a handler for when a row is swapped, update empty regions.
-    tableDrag.row.prototype.onSwap = function (swappedRow) {
-      checkEmptyRegions(table, this);
-    };
+    if (tableDrag.row) {
+      tableDrag.row.prototype.onSwap = function (swappedRow) {
+        checkEmptyRegions(table, this);
+      };
+    }
 
     // A custom message for the blocks page specifically.
     Drupal.theme.tableDragChangedWarning = function () {