Fixed #21912 -- Improved error message if a view returns None.

Thanks brycenesbitt for the report.

django/core/handlers/base.py

@@ -128,7 +128,8 @@ class BaseHandler(object):
                     view_name = callback.__name__
                 else:                                           # CBV
                     view_name = callback.__class__.__name__ + '.__call__'
-                raise ValueError("The view %s.%s didn't return an HttpResponse object." % (callback.__module__, view_name))
+                raise ValueError("The view %s.%s didn't return an HttpResponse object. It returned None instead."
+                                 % (callback.__module__, view_name))
 
             # If the response supports deferred rendering, apply template
             # response middleware and then render the response

tests/middleware_exceptions/tests.py

@@ -382,7 +382,7 @@ class MiddlewareTests(BaseMiddlewareExceptionTest):
         self._add_middleware(middleware)
         self._add_middleware(pre_middleware)
         self.assert_exceptions_handled('/middleware_exceptions/null_view/', [
-            "The view middleware_exceptions.views.null_view didn't return an HttpResponse object.",
+            "The view middleware_exceptions.views.null_view didn't return an HttpResponse object. It returned None instead.",
         ],
             ValueError())
 
@@ -399,7 +399,7 @@ class MiddlewareTests(BaseMiddlewareExceptionTest):
         self._add_middleware(middleware)
         self._add_middleware(pre_middleware)
         self.assert_exceptions_handled('/middleware_exceptions/null_view/', [
-            "The view middleware_exceptions.views.null_view didn't return an HttpResponse object."
+            "The view middleware_exceptions.views.null_view didn't return an HttpResponse object. It returned None instead."
         ],
             ValueError())
 
@@ -693,7 +693,7 @@ class BadMiddlewareTests(BaseMiddlewareExceptionTest):
         self._add_middleware(bad_middleware)
         self._add_middleware(pre_middleware)
         self.assert_exceptions_handled('/middleware_exceptions/null_view/', [
-            "The view middleware_exceptions.views.null_view didn't return an HttpResponse object.",
+            "The view middleware_exceptions.views.null_view didn't return an HttpResponse object. It returned None instead.",
             'Test Response Exception'
         ])
 
@@ -710,7 +710,7 @@ class BadMiddlewareTests(BaseMiddlewareExceptionTest):
         self._add_middleware(bad_middleware)
         self._add_middleware(pre_middleware)
         self.assert_exceptions_handled('/middleware_exceptions/null_view/', [
-            "The view middleware_exceptions.views.null_view didn't return an HttpResponse object."
+            "The view middleware_exceptions.views.null_view didn't return an HttpResponse object. It returned None instead."
         ],
             ValueError())