fix: remove pendaing outbound group session creation on completed or errored

lib/encryption/key_manager.dart

@@ -443,9 +443,12 @@ class KeyManager {
     }
     _pendingNewOutboundGroupSessions[roomId] =
         _createOutboundGroupSession(roomId);
+    try {
       await _pendingNewOutboundGroupSessions[roomId];
+    } finally {
       return _pendingNewOutboundGroupSessions.remove(roomId)!;
     }
+  }
 
   /// Prepares an outbound group session for a given room ID. That is, load it from
   /// the database, cycle it if needed and create it if absent.