Change _validate_layout to accept functions that return layouts.

dash/dash.py

@@ -875,10 +875,12 @@ def _validate_layout(self):
                 'Make sure to set the `layout` attribute of your application '
                 'before running the server.')
 
+        to_validate = self._layout_value()
+
         layout_id = getattr(self.layout, 'id', None)
 
         component_ids = {layout_id} if layout_id else set()
-        for component in self.layout.traverse():
+        for component in to_validate.traverse():
             component_id = getattr(component, 'id', None)
             if component_id and component_id in component_ids:
                 raise exceptions.DuplicateIdError(

tests/test_integration.py

@@ -482,3 +482,14 @@ def test_external_files_init(self):
                 (("//script[@src='{}']", x) for x in js_urls),
                 (("//link[@href='{}']", x) for x in css_urls)):
             self.driver.find_element_by_xpath(fmt.format(url))
+
+    def test_func_layout_accepted(self):
+
+        app = dash.Dash()
+
+        def create_layout():
+            return html.Div('Hello World')
+        app.layout = create_layout
+
+        self.startServer(app)
+        time.sleep(0.5)