eror CRUD Datatable Ajax

1. Saya menggunakan CRUD AJAX Datatable laravel 5.4 namun tidak bisa menyimpan Data ketika mengisi form kemudian klik SUBMIT muncul notif msg success dengan tampilan blank putih tapil tidak bisa redirect ke halaman form kembali. jika saya refresh kembali maka kembali ke halaman form tadi dan data yang di input tadi tersimpan. SS SIMPAN DATA

2. Permasalahan pada form Edit data saat mengedit data kemudian saya klik SUMBIT muncul pesan eror berikut, SS EDIT DATA

Mohon bantuannya terima kasih KATEGORI MODEL

 <?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Kategori extends Model
{
    protected $table = 'kategori';
	protected $primaryKey = 'id_kategori';

	public function produk(){
		return $this->hasMany('App\Produk', 'id_kategori');
	}
}

KATEGORI CONTROLLER

 <?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Kategori;

class KategoriController extends Controller
{
   public function index()
   {
      return view('kategori.index');
   }

   public function listData()
   {

     $kategori = Kategori::orderBy('id_kategori', 'desc')->get();
     $no = 0;
     $data = array();
     foreach($kategori as $list){
       $no ++;
       $row = array();
       $row[] = $no;
       $row[] = $list->nama_kategori;
       $row[] = '<div class="btn-group">
               <a onclick="editForm('.$list->id_kategori.')" class="btn btn-primary btn-sm"><i class="fa fa-pencil"></i></a>
               <a onclick="deleteData('.$list->id_kategori.')" class="btn btn-danger btn-sm"><i class="fa fa-trash"></i></a></div>';
       $data[] = $row;
     }

     $output = array("data" => $data);
     return response()->json($output);
   }

   public function store(Request $request)
   {
      $kategori = new Kategori;
      $kategori->nama_kategori = $request['nama'];
      $kategori->save();
   }

   public function edit($id)
   {
     $kategori = Kategori::find($id);
     echo json_encode($kategori);
   }

   public function update(Request $request, $id)
   {
      $kategori = Kategori::find($id);
      $kategori->nama_kategori = $request['nama'];
      $kategori->update();
   }

   public function destroy($id)
   {
      $kategori = Kategori::find($id);
      $kategori->delete();
   }
}

KATEGORI ROUTE

   Route::get('kategori/data', 'KategoriController@listData')->name('kategori.data');
  Route::resource('kategori', 'KategoriController');

VIEW DATA KATEGORI

@extends('layouts.app')

@section('title')
  Daftar Kategori
@endsection

@section('breadcrumb')
   @parent
   <li>kategori</li>
@endsection

@section('content')
<div class="row">
  <div class="col-xs-12">
    <div class="box">
      <div class="box-header">
        <a onclick="addForm()" class="btn btn-success"><i class="fa fa-plus-circle"></i> Tambah</a>
      </div>
      <div class="box-body">

<table class="table table-striped">
<thead>
   <tr>
      <th width="30">No</th>
      <th>Nama Kategori</th>
      <th width="100">Aksi</th>
   </tr>
</thead>
<tbody></tbody>
</table>

      </div>
    </div>
  </div>
</div>

@include('kategori.form')
@endsection

@section('script')
<script type="text/javascript">
var table, save_method;
$(function(){
   table = $('.table').DataTable({
     "processing" : true,
     "ajax" : {
       "url" : "{{ route('kategori.data') }}",
       "type" : "GET"
     }
   });

   $('#modal-form form').validator().on('submit', function(e){
      if(!e.isDefaultPrevented()){
         var id = $('#id').val();
         if(save_method == "add") url = "{{ route('kategori.store') }}";
         else url = "kategori/"+id;

         $.ajax({
           url : url,
           type : "POST",
           data : $('#modal-form form').serialize(),
           success : function(data){
             $('#modal-form').modal('hide');
             table.ajax.reload();
           },
           error : function(){
             alert("Tidak dapat menyimpan data!");
           }
         });
         return false;
     }
   });
});

function addForm(){
   save_method = "add";
   $('input[name=_method]').val('POST');
   $('#modal-form').modal('show');
   $('#modal-form form')[0].reset();
   $('.modal-title').text('Tambah Kategori');
}

function editForm(id){
   save_method = "edit";
   $('input[name=_method]').val('PATCH');
   $('#modal-form form')[0].reset();
   $.ajax({
     url : "kategori/"+id+"/edit",
     type : "GET",
     dataType : "JSON",
     success : function(data){
       $('#modal-form').modal('show');
       $('.modal-title').text('Edit Kategori');

       $('#id').val(data.id_kategori);
       $('#nama').val(data.nama_kategori);

     },
     error : function(){
       alert("Tidak dapat menampilkan data!");
     }
   });
}

function deleteData(id){
   if(confirm("Apakah yakin data akan dihapus?")){
     $.ajax({
       url : "kategori/"+id,
       type : "POST",
       data : {'_method' : 'DELETE', '_token' : $('meta[name=csrf-token]').attr('content')},
       success : function(data){
         table.ajax.reload();
       },
       error : function(){
         alert("Tidak dapat menghapus data!");
       }
     });
   }
}
</script>
@endsection

FORM INPUT DATA KATEGORI

<div class="modal" id="modal-form" tabindex="-1" role="dialog" aria-hidden="true" data-backdrop="static">
   <div class="modal-dialog modal-lg">
      <div class="modal-content">

   <form class="form-horizontal" data-toggle="validator" method="post">
   {{ csrf_field() }} {{ method_field('POST') }}

   <div class="modal-header">
      <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true"> × </span> </button>
      <h3 class="modal-title"></h3>
   </div>

<div class="modal-body">

   <input type="hidden" id="id" name="id">
   <div class="form-group">
      <label for="nama" class="col-md-3 control-label">Nama Produk</label>
      <div class="col-md-6">
         <input id="nama" type="text" class="form-control" name="nama" autofocus required>
         <span class="help-block with-errors"></span>
      </div>
   </div>

</div>

   <div class="modal-footer">
      <button type="submit" class="btn btn-primary btn-save"><i class="fa fa-floppy-o"></i> Simpan </button>
      <button type="button" class="btn btn-warning" data-dismiss="modal"><i class="fa fa-arrow-circle-left"></i> Batal</button>
   </div>

   </form>

         </div>
      </div>
   </div>