simplify test_simple_router not rely on _next_view_id.

Author: Archmonger

tests/test_router.py

@@ -45,17 +45,10 @@ def sample():
 
     await display.goto("/missing")
 
-    try:
-        root_element = await display.root_element()
-    except AttributeError:
-        root_element = await display.page.wait_for_selector(
-            f"#display-{display._next_view_id}",  # type: ignore
-            state="attached",
-        )
+    root_element = await display.page.wait_for_selector("#app", state="attached")
 
     assert not await root_element.inner_html()
 
-
 async def test_nested_routes(display: DisplayFixture):
     @component
     def sample():