[PR #7333/3a54d378 backport][3.8] Fix TLS transport is None error (#7357)

Co-authored-by: Sam Bull <[email protected]>
Fixes #3355

Author: patchback[bot]

CHANGES/3355.bugfix

@@ -0,0 +1 @@
+Fixed a transport is :data:`None` error -- by :user:`Dreamsorcerer`.

aiohttp/connector.py

@@ -1121,6 +1121,9 @@ async def _start_tls_connection(
                 f"[{type_err!s}]"
             ) from type_err
         else:
+            if tls_transport is None:
+                msg = "Failed to start TLS (possibly caused by closing transport)"
+                raise client_error(req.connection_key, OSError(msg))
             tls_proto.connection_made(
                 tls_transport
             )  # Kick the state machine of the new TLS protocol