Fix lerping for `NavigationRailThemeData` icon themes (#120066)

* Fix lerping for NavigationRail icon themes

* fix typo

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

@@ -151,8 +151,10 @@ class NavigationRailThemeData with Diagnosticable {
       elevation: lerpDouble(a?.elevation, b?.elevation, t),
       unselectedLabelTextStyle: TextStyle.lerp(a?.unselectedLabelTextStyle, b?.unselectedLabelTextStyle, t),
       selectedLabelTextStyle: TextStyle.lerp(a?.selectedLabelTextStyle, b?.selectedLabelTextStyle, t),
-      unselectedIconTheme: IconThemeData.lerp(a?.unselectedIconTheme, b?.unselectedIconTheme, t),
-      selectedIconTheme: IconThemeData.lerp(a?.selectedIconTheme, b?.selectedIconTheme, t),
+      unselectedIconTheme: a?.unselectedIconTheme == null && b?.unselectedIconTheme == null
+        ? null : IconThemeData.lerp(a?.unselectedIconTheme, b?.unselectedIconTheme, t),
+      selectedIconTheme: a?.selectedIconTheme == null && b?.selectedIconTheme == null
+        ? null : IconThemeData.lerp(a?.selectedIconTheme, b?.selectedIconTheme, t),
       groupAlignment: lerpDouble(a?.groupAlignment, b?.groupAlignment, t),
       labelType: t < 0.5 ? a?.labelType : b?.labelType,
       useIndicator: t < 0.5 ? a?.useIndicator : b?.useIndicator,

packages/flutter/test/material/navigation_rail_theme_test.dart

@@ -227,6 +227,14 @@ void main() {
     expect(_indicatorDecoration(tester)?.color, indicatorColor);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/118618.
+  testWidgets('NavigationRailThemeData lerps correctly with null iconThemes', (WidgetTester tester) async {
+    final NavigationRailThemeData lerp = NavigationRailThemeData.lerp(const NavigationRailThemeData(), const NavigationRailThemeData(), 0.5)!;
+
+    expect(lerp.selectedIconTheme, isNull);
+    expect(lerp.unselectedIconTheme, isNull);
+  });
+
   testWidgets('Default debugFillProperties', (WidgetTester tester) async {
     final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
     const NavigationRailThemeData().debugFillProperties(builder);