changed typecheck for list value to use isinstance instead of type

dash/dash.py

@@ -2342,17 +2342,21 @@ def update(pathname_, search_, **states):
 
             # Set validation_layout
             if not self.config.suppress_callback_exceptions:
-                self.validation_layout = html.Div(
-                    [
-                        page["layout"]() if callable(page["layout"]) else page["layout"]
-                        for page in _pages.PAGE_REGISTRY.values()
-                    ]
-                    + [
+                layout = self.layout
+                if not isinstance(layout, list):
+                    layout = [
                         # pylint: disable=not-callable
                         self.layout()
                         if callable(self.layout)
                         else self.layout
                     ]
+
+                self.validation_layout = html.Div(
+                    [
+                        page["layout"]() if callable(page["layout"]) else page["layout"]
+                        for page in _pages.PAGE_REGISTRY.values()
+                    ]
+                    + layout
                 )
                 if _ID_CONTENT not in self.validation_layout:
                     raise Exception("`dash.page_container` not found in the layout")