The argument type 'String' can't be assigned to the parameter type 'AllPosts'

I'm not sure why it can't saveform. Please help me QAQ this is messing up my life

This is the error: lib/navpages/upload/edit_post_screen.dart:98:65: Error: The argument type 'String' can't be assigned to the parameter type 'AllPosts'.

• 'AllPosts' is from 'package:pinsta/providers/all_posts.dart' ('lib/providers/all_posts.dart'). onPressed:() => saveForm(i,caption!), ^

   import 'package:flutter/material.dart';
   import 'package:pinsta/models/post.dart';
   import 'package:pinsta/providers/all_posts.dart';
   import 'package:provider/provider.dart';
  
  
   class EditPostScreen extends StatefulWidget {
     static String routeName = '/edit-post';
  
     @override
     State<EditPostScreen> createState() => _EditPostScreenState();
   }
  
   class _EditPostScreenState extends State<EditPostScreen> {
     var form = GlobalKey<FormState>();
  
     String? caption?? ;
  
  
  
     void saveForm(int i, AllPosts myPosts) {
       bool isValid = form.currentState!.validate();
       if (isValid) {
         form.currentState!.save();
         print(caption);
  
         myPosts.updatePost(i,caption);
  
         FocusScope.of(context).unfocus();
         form.currentState!.reset();
         caption = null;
  
         ScaffoldMessenger.of(context).showSnackBar(
           SnackBar(
             content: Row(
               children: [
                 Flexible(child: Text('updated', style: TextStyle(color: Colors.white, fontSize: 14.5))),
               ],
             ),
             duration: Duration(seconds: 1),
           ),
         );
  
       }
     }
  
  
     @override
     Widget build(BuildContext context) {
       AllPosts postList1 = Provider.of <AllPosts> (context);
       int i = ModalRoute.of(context)?.settings.arguments as int;
  
       return Scaffold(
         resizeToAvoidBottomInset: false,
         body: SafeArea(
           child: Padding(
             padding: EdgeInsets.symmetric(horizontal: 12.0,vertical: 18.0),
             child: Column(
               crossAxisAlignment: CrossAxisAlignment.center,
               children: [
                 Image.asset('assets/images/upload_image.png',height: 200,
                   width: 200,),
                 SizedBox(height: 50.0,),
                 Form(
                   key: form,
                   child: Column(
                     children: [
                       SizedBox(
                         height: 60.0,
                       ),
                       TextFormField(
                         decoration: InputDecoration(
                             prefixIcon: Icon(Icons.people, color: Colors.grey),
                             hintText: 'caption',
                             border: OutlineInputBorder(
                                 borderRadius: BorderRadius.circular(24.0)
                             )
                         ),
                         validator: (value) {
                           if (value == null)
                             return caption;
                         },
                         onSaved: (value) {caption = value;},
                       ),
                       SizedBox(
                         height: 50.0,
                       ),
                       Padding(
                         padding: const EdgeInsets.symmetric(horizontal: 25.0),
                         child: Container(
                             padding: EdgeInsets.all(5),
                             decoration: BoxDecoration(
                               color: Colors.redAccent,
                               borderRadius: BorderRadius.circular(12),
                             ),
                             child: Center(
                               child: TextButton(
                                 onPressed:() => saveForm(i,caption!),
                                 child: Text(
                                   'Update', style: TextStyle(fontSize: 17, fontWeight: FontWeight.bold, color: Colors.white),
                                 ),
                               ),
                             )
                         ),
                       ),
                     ],
                   ),
                 ),
               ],
             ),
           ),
         ),
       );
     }
   }
  

This is the provider:

    import 'package:flutter/material.dart';
    import '../models/post.dart';
    
    class AllPosts with ChangeNotifier {
      List <Post> myPosts = [];
    
      List<Post> getMyPosts() {
        return myPosts;
      }
    
      void addPost(imageUrl, caption, username) {
        myPosts.insert(0, Post(imageUrl: imageUrl, caption: caption, username:username));
        notifyListeners();
      }
    
      void removePost(i) {
        myPosts.removeAt(i);
        notifyListeners();
      }
    
      void updatePost(i,caption ) {
        myPosts[i].caption = caption;
        notifyListeners();
      }
    }