Remove null session interface, fix incorrect runtime error

Previously failure to set session type resulted in error message that says a secret key is required

src/flask_session/__init__.py

@@ -3,7 +3,6 @@
     FileSystemSessionInterface,
     MemcachedSessionInterface,
     MongoDBSessionInterface,
-    NullSessionInterface,
     RedisSessionInterface,
     SqlAlchemySessionInterface,
 )
@@ -150,6 +149,6 @@ def _get_interface(self, app):
                 bind_key=SESSION_SQLALCHEMY_BIND_KEY,
             )
         else:
-            session_interface = NullSessionInterface()
+            raise RuntimeError(f"Unrecognized value for SESSION_TYPE: {SESSION_TYPE}")
 
         return session_interface

src/flask_session/sessions.py

@@ -79,18 +79,6 @@ def _sign(self, app, sid):
         return signer.sign(sid_as_bytes).decode("utf-8")
 
 
-class NullSessionInterface(SessionInterface):
-    """Used to open a :class:`flask.sessions.NullSession` instance.
-
-    If you do not configure a different ``SESSION_TYPE``, this will be used to
-    generate nicer error messages.  Will allow read-only access to the empty
-    session but fail on setting.
-    """
-
-    def open_session(self, app, request):
-        return None
-
-
 class ServerSideSessionInterface(SessionInterface, ABC):
     """Used to open a :class:`flask.sessions.ServerSideSessionInterface` instance."""