reverted to fix the issue without changing setting

rest_framework/schemas.py

@@ -600,7 +600,8 @@ class SchemaGenerator(object):
         if not is_list_view(path, method, view):
             return []
 
-        if not getattr(view, 'pagination_class', None):
+        pagination = getattr(view, 'pagination_class', None)
+        if not pagination or not pagination.page_size:
             return []
 
         paginator = view.pagination_class()

rest_framework/settings.py

@@ -50,7 +50,7 @@ DEFAULTS = {
     'DEFAULT_VERSIONING_CLASS': None,
 
     # Generic view behavior
-    'DEFAULT_PAGINATION_CLASS': None,
+    'DEFAULT_PAGINATION_CLASS': 'rest_framework.pagination.PageNumberPagination',
     'DEFAULT_FILTER_BACKENDS': (),
 
     # Throttling