Fix textbook module.

common/lib/xmodule/xmodule/textbook_module.py

@@ -3,11 +3,14 @@ from xmodule.xml_module import XmlDescriptor
 from lxml import etree
 
 class TextbookModule(XModule):
-    def __init__(self, system, location, definition, instance_state=None,
+    def __init__(self, system, location, definition, descriptor, instance_state=None,
                  shared_state=None, **kwargs):
-        XModule.__init__(self, system, location, definition,
+        XModule.__init__(self, system, location, definition, descriptor,
                          instance_state, shared_state, **kwargs)
 
+    def get_display_items(self):
+      return []
+
 class TextbookDescriptor(XmlDescriptor):
 
     module_class = TextbookModule