Fix issue #4312 add is_iterable() check in grid. (#4853)

Author: kiatng

app/design/adminhtml/default/default/template/widget/grid.phtml

@@ -146,7 +146,7 @@ $numColumns = count($this->getColumns());
 
             <?php endforeach ?>
             </tr>
-            <?php if ($_multipleRows = $this->getMultipleRows($_item)):?>
+            <?php if (is_iterable($_multipleRows = $this->getMultipleRows($_item))):?>
                 <?php foreach ($_multipleRows as $_i):?>
                 <tr>
                     <?php $i=0;foreach ($this->getMultipleRowColumns($_i) as $_column): ?>