fixed warning by setting assert

src/pyside6/trade_widget.py

@@ -62,6 +62,7 @@ class TradeCollection(QWidget):
                  parent):
         super(TradeCollection, self).__init__()
         self.parent = parent
+        assert message.trade
         self.unique_item = message.trade.unique_item()
         self.main_layout = QVBoxLayout()
         self.setLayout(self.main_layout)