Add dict instance check before accessing it

changelog.d/15944.bugfix

@@ -0,0 +1 @@
+Add dic instance check before accessing auth session.

synapse/handlers/auth.py

@@ -483,7 +483,7 @@ async def check_ui_auth(
 
         sid: Optional[str] = None
         authdict = clientdict.pop("auth", {})
-        if "session" in authdict:
+        if isinstance(authdict, dict) and "session" in authdict:
             sid = authdict["session"]
 
         # Convert the URI and method to strings.