Fix Internal Server Error on GET /saml2/authn_response (#9623)

* Fix Internal Server Error on `GET /saml2/authn_response`

Seems to have been introduced in #8765 (Synapse 1.24.0)

* Fix newsfile

changelog.d/9623.bugfix

@@ -0,0 +1 @@
+Fix Internal Server Error on `GET /_synapse/client/saml2/authn_response` request.

synapse/rest/synapse/client/saml2/response_resource.py

@@ -14,24 +14,30 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import TYPE_CHECKING
+
 from synapse.http.server import DirectServeHtmlResource
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 
 class SAML2ResponseResource(DirectServeHtmlResource):
     """A Twisted web resource which handles the SAML response"""
 
     isLeaf = 1
 
-    def __init__(self, hs):
+    def __init__(self, hs: "HomeServer"):
         super().__init__()
         self._saml_handler = hs.get_saml_handler()
+        self._sso_handler = hs.get_sso_handler()
 
     async def _async_render_GET(self, request):
         # We're not expecting any GET request on that resource if everything goes right,
         # but some IdPs sometimes end up responding with a 302 redirect on this endpoint.
         # In this case, just tell the user that something went wrong and they should
         # try to authenticate again.
-        self._saml_handler._render_error(
+        self._sso_handler.render_error(
             request, "unexpected_get", "Unexpected GET request on /saml2/authn_response"
         )