possible fix for exception on mount

mkdocs.yml

@@ -178,9 +178,8 @@ plugins:
     default_handler: python
     handlers:
         python:
-          rendering:
-            show_source: false
-          selection:
+          options:
+            show_source: false          
             filters:
               - "!^_"
               - "^__init__$"

src/textual/widget.py

@@ -329,7 +329,7 @@ class Widget(DOMNode):
 
         """
         self.app._register(self, *anon_widgets, **widgets)
-        self.screen.refresh(layout=True)
+        self.app.screen.refresh(layout=True)
 
     def compose(self) -> ComposeResult:
         """Called by Textual to create child widgets.