Merge pull request silverstripe#4473 from kinglozzer/gf-sortableheade…

…r-method-exists

Check that method 'hasMethod' exists in GridFieldSortableHeader

forms/gridfield/GridFieldSortableHeader.php

@@ -111,7 +111,7 @@ public function getHTMLFragments($gridField) {
 					if($tmpItem instanceof SS_List) {
 						// It's impossible to sort on a HasManyList/ManyManyList
 						break;
-					} elseif($tmpItem->hasMethod($methodName)) {
+					} elseif(method_exists($tmpItem, 'hasMethod') && $tmpItem->hasMethod($methodName)) {
 						// The part is a relation name, so get the object/list from it
 						$tmpItem = $tmpItem->$methodName();
 					} elseif($tmpItem instanceof DataObject && $tmpItem->hasField($methodName)) {