fix to multi-class for ss4 support

Author: John Milmine

src/GridFieldAddNewMultiClass.php

@@ -6,7 +6,7 @@
 use SilverStripe\Control\HTTPResponse_Exception;
 use SilverStripe\Core\ClassInfo;
 use SilverStripe\Core\Config\Config;
-use SilverStripe\Core\Object;
+use SilverStripe\Core\Injector\Injector;
 use SilverStripe\Forms\DropdownField;
 use SilverStripe\Forms\GridField\GridField;
 use SilverStripe\Forms\GridField\GridField_HTMLProvider;
@@ -196,7 +196,7 @@ public function handleAdd($grid, $request)
             throw new HTTPResponse_Exception(400);
         }
 
-        $handler = Object::create(
+        $handler = Injector::inst()->create(
             $this->itemRequestClass,
             $grid,
             $component,