Bottom navigation items length docs (#72490)

packages/flutter/lib/src/material/bottom_navigation_bar.dart

@@ -50,6 +50,9 @@ enum BottomNavigationBarType {
 /// [BottomNavigationBarType.fixed] when there are less than four items, and
 /// [BottomNavigationBarType.shifting] otherwise.
 ///
+/// The length of [items] must be at least two and each item's icon and title/label
+/// must not be null.
+///
 ///  * [BottomNavigationBarType.fixed], the default when there are less than
 ///    four [items]. The selected item is rendered with the
 ///    [selectedItemColor] if it's non-null, otherwise the theme's