fix: use default value when MaterialLocalizations are not available

 import 'dart:async';
 
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 
 import '../modal_bottom_sheet.dart';
@@ -33,21 +34,19 @@ class _ModalBottomSheet<T> extends StatefulWidget {
 
 class _ModalBottomSheetState<T> extends State<_ModalBottomSheet<T>> {
   String _getRouteLabel() {
-    if (Theme.of(context, shadowThemeOnly: true) == null) {
-      return '';
-    } else {
-      final platform = Theme.of(context).platform;
+    final platform = Theme.of(context)?.platform ?? defaultTargetPlatform;
     switch (platform) {
       case TargetPlatform.iOS:
         return '';
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
-          assert(debugCheckHasMaterialLocalizations(context));
+        if (Localizations.of(context, MaterialLocalizations) != null) {
           return MaterialLocalizations.of(context).dialogLabel;
-          break;
+        } else {
+          return DefaultMaterialLocalizations().dialogLabel;
         }
-      return null;
     }
+    return null;
   }
 
   @override