Fix issue with validating variable panel types on Views (#568)

lumen/views/base.py

@@ -153,7 +153,7 @@ def _valid_keys_(cls) -> List[str] | None:
             panel_type = cls._panel_type
         except Exception:
             panel_type = None
-        if panel_type is None:
+        if panel_type is None or isinstance(panel_type, property):
             return None
         valid = super()._valid_keys_
         return valid + list(panel_type.param)