Laravel 8 Error: Attempt to assign property "id_gerai" on null - php

Question

I building a page to insert rows into database tables using a HTML form. I created my view, model, controller, and route already and it return this error when i try to submit the form:
Attempt to assign property "id_gerai" on null
Here are the rest of the code
resources\views\dashboard\manajemen_donasi.blade.php
#extends('layouts.dashboard')
#section('title')
<title>KitaKenyang - Dashboard Admin</title>
#endsection
#section('nav_link')
<li class="nav-item active">
<a class="nav-link" href="/manajemen donasi">
<i class="fas fa-fw fa-tachometer-alt"></i>
<span>Manajemen Donasi</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="/manajemen penyimpanan">
<i class="fas fa-fw fa-tachometer-alt"></i>
<span>Manajemen Penyimpanan</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="/manajemen penyaluran">
<i class="fas fa-fw fa-tachometer-alt"></i>
<span>Manajemen Penyaluran</span></a>
</li>
#endsection
#section('username')
Admin
#endsection
#section('content')
<!-- Page Heading -->
<div class="d-sm-flex align-items-center justify-content-between mb-4">
<h1 class="h3 mb-0 text-gray-800 text-center">Dashboard Manajemen Donasi</h1>
</div>
<div class="container-fluid">
<!-- DataTales Example -->
<div class="card shadow mb-4">
<div class="card-header py-3">
<h6 class="m-0 font-weight-bold text-success">Input Donasi</h6>
</div>
<div class="card-body">
<form action="#" method="post" class="probootstrap-form">
#csrf <!-- {{ csrf_field() }} -->
<div class="form-group">
<label for="id_user">ID Donatur</label>
<input type="text" class="form-control" id="id_user" name="id_user">
</div>
<div class="form-group">
<label for="tanggal">Tanggal Donasi</label>
<input type="date" class="form-control" id="tanggal" name="tanggal">
</div>
<div class="form-group">
<label for="id_kategori_barang">Kategori</label>
<input type="text" class="form-control" id="id_kategori_barang" name="id_kategori_barang">
<i>* dry staple: 1 | dehydrated food: 2 | canned food: 3 | frozen food: 4</i>
</div>
<div class="form-group">
<label for="id_kuantitas">Kuantitas</label>
<input type="text" class="form-control" id="kuantitas" name="kuantitas">
</div>
<div class="form-group">
<label for="id_nama_barang">Nama barang</label>
<input type="text" class="form-control" id="nama_barang" name="nama_barang">
</div>
<div class="form-group">
<label for="id_gerai">ID Gerai</label>
<input type="text" class="form-control" id="id_gerai" name="id_gerai">
</div>
<div class="form-group">
<input type="submit" class="btn btn-success btn-lg" id="submit" name="submit" value="Input">
</div>
</form>
</div>
</div>
</div>
<!-- /.container-fluid -->
#endsection
routes\web.php
<?php
use Illuminate\Http\Request;
use Illuminate\Support\Facades\Route;
use App\Models\Donasi;
/*
|--------------------------------------------------------------------------
| Web Routes
|--------------------------------------------------------------------------
|
| Here is where you can register web routes for your application. These
| routes are loaded by the RouteServiceProvider within a group which
| contains the "web" middleware group. Now create something great!
|
*/
Route::get('/', 'PagesController#index');
Route::get('/beranda', 'PagesController#index');
Route::get('/tentang kami', 'PagesController#about');
Route::get('/berita', 'PagesController#news');
Route::get('/program', 'PagesController#program');
Route::get('/gerai donasi', 'PagesController#gerai');
Route::get('/login', 'PagesController#login');
Route::get('/register', 'PagesController#register');
Route::get('/kinerja organisasi', 'DashboardController#laporan_org');
Route::get('/donasi user', 'DashboardController#laporan_user');
Route::get('/profil user', 'DashboardController#profil_user');
Route::get('/manajemen donasi', 'DashboardController#manajemen_donasi');
Route::post('manajemen donasi', 'DashboardController#input_donasi');
Route::get('/manajemen penyimpanan', 'DashboardController#manajemen_penyimpanan');
Route::get('/manajemen penyaluran', 'DashboardController#manajemen_penyaluran');
App\Models\Donasi.php
<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
class Donasi extends Model
{
use HasFactory;
protected $table = 'donasi';
protected $primaryKey = 'id';
protected $fillable = [
'id_user',
'id_kategori_barang',
'id_gerai',
'nama_barang',
'kuantitas',
'tanggal',
];
public $timestamps = false;
}
app\Http\Controllers\DashboardController
<?php
namespace App\Http\Controllers;
use Illuminate\Http\Request;
use App\Models\Program;
use App\Models\DonasiDalamPenyimpanan;
use App\Models\TotalDonasi;
use App\Models\TotalDonatur;
use App\Models\Donasi;
class DashboardController extends Controller
{
public function laporan_org()
{
$totaldonatur = TotalDonatur::all();
$totaldonasi = TotalDonasi::all();
$program = Program::all();
return view('dashboard.laporan_org', compact('program', 'totaldonasi', 'totaldonatur'));
}
public function laporan_user()
{
return view('dashboard.laporan_user');
}
public function profil_user()
{
return view('dashboard.profil_user');
}
public function manajemen_donasi()
{
return view('dashboard.manajemen_donasi');
}
public function manajemen_penyimpanan()
{
$penyimpanan = DonasiDalamPenyimpanan::all();
return view('dashboard.manajemen_penyimpanan', compact('penyimpanan'));
}
public function manajemen_penyaluran()
{
$program = Program::all();
return view('dashboard.manajemen_penyaluran', compact('program'));
}
public function input_donasi(Request $request)
{
$donasi = new Donasi;
$donasi->id_user = $request->input('id_user');
$donasi->tanggal = $request->input('tanggal');
$donasi->id_kategori_barang = $request->input('id_kategori_barang');
$donasi->kuantitas = $request->input('kuantitas');
$donasi->nama_barang = $request->input('nama_barang');
$donasiModel->id_gerai = $request->input('id_gerai');
$donasi->save();
}
}