Declare NavigationRailDestination.label as non-nullable (#77427)

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

@@ -490,7 +490,7 @@ class _NavigationRailState extends State<NavigationRail> with TickerProviderStat
                           extendedTransitionAnimation: _extendedAnimation,
                           selected: widget.selectedIndex == i,
                           icon: widget.selectedIndex == i ? widget.destinations[i].selectedIcon : widget.destinations[i].icon,
-                          label: widget.destinations[i].label!,
+                          label: widget.destinations[i].label,
                           destinationAnimation: _destinationAnimations[i],
                           labelType: labelType,
                           iconTheme: widget.selectedIndex == i ? selectedIconTheme : unselectedIconTheme,
@@ -818,10 +818,11 @@ class NavigationRailDestination {
   const NavigationRailDestination({
     required this.icon,
     Widget? selectedIcon,
-    this.label,
+    required this.label,
     this.padding,
   }) : selectedIcon = selectedIcon ?? icon,
-       assert(icon != null);
+       assert(icon != null),
+       assert(label != null);
 
   /// The icon of the destination.
   ///
@@ -856,7 +857,7 @@ class NavigationRailDestination {
   /// [NavigationRail.labelType] is [NavigationRailLabelType.none], the label is
   /// still used for semantics, and may still be used if
   /// [NavigationRail.extended] is true.
-  final Widget? label;
+  final Widget label;
 
   /// The amount of space to inset the destination item.
   final EdgeInsetsGeometry? padding;