Bug fix: Reference to __singlePageContent in page_layout

nicegui/page_layout.py

@@ -272,7 +272,7 @@ def __init__(self, position: PageStickyPositions = 'bottom-right', x_offset: flo
 
 def _check_current_slot(element: Element) -> None:
     parent = context.get_slot().parent
-    if parent != parent.client.content and parent != parent.client.state.get("__singlePageContent", None):
+    if parent != parent.client.content and parent != parent.client.single_page_content:
         log.warning(f'Found top level layout element "{element.__class__.__name__}" inside element "{parent.__class__.__name__}". '
                     'Top level layout elements should not be nested but must be direct children of the page content. '
                     'This will be raising an exception in NiceGUI 1.5')  # DEPRECATED