make sure signal is a SignalInstance

src/psygnal/_group_descriptor.py

@@ -314,7 +314,9 @@ def _setattr_and_emit_(self: object, name: str, value: Any) -> None:
 
             sig_name = f"{name}{signal_suffix}"
             group: SignalGroup | None = getattr(self, signal_group_name, None)
-            signal: SignalInstance | None = group.signals.get(sig_name, None)
+            signal: SignalInstance | None = (
+                None if group is None else group.signals.get(sig_name, None)
+            )
             # don't emit if the signal doesn't exist or has no listeners
             if group is None or signal is None or len(signal) < 2 and not len(group):
                 return super_setattr(self, name, value)