fix: book category (handle null)

lib/src/models/book.dart

+import "package:library_app/src/models/category.dart";
+
 class Book {
   int id;
   String title;
@@ -10,13 +12,27 @@ class Book {
       this.category);
 
   factory Book.fromJson(Map<String, dynamic> data) {
+    if (data["category_detail"] != null) {
+      final Category category = Category.fromJson(
+        data["category_detail"],
+      );
+      return Book(
+        data["id"],
+        data["title"],
+        data["author"],
+        data["description"],
+        data["cover_image"],
+        category.name,
+      );
+    }
+
     return Book(
       data['id'] as int,
       data['title'] as String,
       data['author'] as String,
       data['description'] as String,
       data['coverUrl'] as String?,
-      data['category'] as String?,
+      null,
     );
   }
 }

lib/src/models/category.dart

@@ -2,6 +2,12 @@ class Category {
   String name;
 
   Category(this.name);
+
+  factory Category.fromJson(Map<String, dynamic> data) {
+    return Category(
+      data['name'] as String,
+    );
+  }
 }
 
 final initialCategories = [

lib/src/providers/book_provider.dart

@@ -50,4 +50,47 @@ class BookProvider with ChangeNotifier {
       debugPrint("Error: Fetch books failed, $error");
     }
   }
+
+  Future<void> getCategories() async {
+    try {
+      final response = await http.get(
+        Uri.parse('$baseUrl/categories'),
+        headers: {'Content-Type': 'application/json'},
+      );
+
+      if (response.statusCode == 200) {
+        final data = jsonDecode(response.body);
+        books = data["results"];
+      } else {
+        final code = response.statusCode;
+        debugPrint("Error: Fetch books failed, $code");
+      }
+
+      notifyListeners();
+    } catch (error) {
+      debugPrint("Error: Fetch books failed, $error");
+    }
+  }
+
+  Future<void> getCategoryById(int id) async {
+    try {
+      final response = await http.get(
+        Uri.parse('$baseUrl/categories/$id'),
+        headers: {'Content-Type': 'application/json'},
+      );
+
+      if (response.statusCode == 200) {
+        final data = jsonDecode(response.body);
+        var category = data["results"];
+        return category;
+      } else {
+        final code = response.statusCode;
+        debugPrint("Error: Fetch books failed, $code");
+      }
+
+      notifyListeners();
+    } catch (error) {
+      debugPrint("Error: Fetch books failed, $error");
+    }
+  }
 }

lib/src/widgets/books/book_detail.dart

@@ -40,14 +40,22 @@ class _BookDetail extends State<BookDetail> {
               textAlign: TextAlign.center,
             ),
           ),
-          Text(
+          Padding(
+            padding: const EdgeInsets.symmetric(vertical: 8.0),
+            child: Text(
               'By ${book.author}',
               style: Theme.of(context).textTheme.labelMedium,
               textAlign: TextAlign.center,
             ),
-          Text(
+          ),
+          Offstage(
+            offstage: book.category == null,
+            child: Badge(
+              padding: const EdgeInsets.symmetric(horizontal: 10.0),
+              label: Text(
                 book.category ?? "",
-            style: Theme.of(context).textTheme.labelSmall,
+              ),
+            ),
           ),
           Padding(
             padding: const EdgeInsets.symmetric(vertical: 10.0),

lib/src/widgets/books/book_item.dart

@@ -63,10 +63,15 @@ class BookItem extends StatelessWidget {
                         ),
                       ],
                     ),
-                    Text(
+                    Offstage(
+                      offstage: _book.category == null,
+                      child: Badge(
+                        padding: const EdgeInsets.symmetric(horizontal: 10.0),
+                        label: Text(
                           _book.category ?? "",
-                      style: Theme.of(context).textTheme.labelMedium,
                         ),
+                      ),
+                    )
                   ],
                 ),
               ),

lib/src/widgets/books/book_list.dart

 import 'package:flutter/material.dart';
+import 'package:library_app/src/models/category.dart';
 import 'package:library_app/src/providers/book_provider.dart';
 
 import 'package:library_app/src/widgets/books/book_item.dart';
@@ -25,16 +26,30 @@ class _BookList extends State<BookList> {
     return Consumer<BookProvider>(
       builder: (context, bookProvider, child) {
         if (bookProvider.books != null) {
-          final Iterable<Book> books = bookProvider.books!.map(
-            (book) => Book(
+          final Iterable<Book> books = bookProvider.books!.map((book) {
+            if (book["category_detail"] != null) {
+              final Category category = Category.fromJson(
+                book["category_detail"],
+              );
+              return Book(
                 book["id"],
                 book["title"],
                 book["author"],
                 book["description"],
                 book["cover_image"],
-              book["category"],
-            ),
+                category.name,
               );
+            }
+
+            return Book(
+              book["id"],
+              book["title"],
+              book["author"],
+              book["description"],
+              book["cover_image"],
+              null,
+            );
+          });
 
           return NestedScrollView(
             headerSliverBuilder: