Minta Tolong, saya membuat update data, error Undefined index ?

<?php
$id = $_GET['id'];

$ambil = $koneksi->query("SELECT * FROM buku WHERE id_buku='$id'");
while ($row = $ambil->fetch_assoc()) {

    // echo "<pre>";
    // print_r($row);
    // echo "</pre>";
    if (isset($_POST["ubah"])) {

        //cek apakah data berhasil di ubah atau tidak
        if (ubah_buku($_POST) > 0) {
            echo "<script>alert('Data Buku Berhasil Di Ubah');</script>";
            echo "<script>location='index.php?halaman=ubah_buku';</script>";
        } else {
            echo "
        <script>
            alert('data gagal di ubah!');
            window.location.href='index.php?halaman=ubah_buku&id';
        </script>
        " . mysqli_error($koneksi);
        }
    }
?>

    <a href="index.php?halaman=buku"><i class="fas fa-arrow-alt-circle-left fa-2x" style="color:red"></i></a>

    <div class="card card-primary">
        <div class="card-header">
            <h3 class="card-title">Ubah Data Buku</h3>
        </div>
        <!-- /.card-header -->
        <!-- form start -->

        <form role="form" action="" method="post" enctype="multipart/form-data">
            <div class="card-body">
                <div class="form-group">
                    <label for="exampleInputText1">Judul Buku</label>
                    <input type="text" class="form-control" placeholder="Judul Buku" name="judul_buku" value="<?php echo $row['judul_buku']; ?>">
                    <label for="exampleInputText1">Penulis Buku</label>
                    <input type="text" class="form-control" placeholder="Penulis Buku" name="penulis_buku" value="<?php echo $row['penulis_buku']; ?>">
                    <label for="exampleInputText1">ISBN Buku</label>
                    <input type="text" class="form-control" placeholder="ISBN" name="isbn_buku" value="<?php echo $row['isbn_buku']; ?>">
                    <label for="exampleInputText1">Jumlah Halaman</label>
                    <input type="number" class="form-control" placeholder="Jumlah Halaman" name="halaman_buku" value="<?php echo $row['halaman_buku']; ?>">
                    <label for="exampleInputText1">Kategori Buku</label>
                    <input type="text" class="form-control" placeholder="Kategori Buku" name="kategori_buku" value="<?php echo $row['kategori_buku']; ?>">
                    <label for="exampleInputText1">Deskripsi Buku</label>
                    <textarea class="form-control" id="exampleFormControlTextarea1" rows="5" placeholder="Deskripsi Buku" name="deskripsi_buku"><?php echo $row['deskripsi_buku']; ?></textarea>
                    <label for="exampleInputText1">Harga Buku</label>
                    <input type="number" class="form-control" placeholder="Harga Buku" name="harga_buku" value="<?php echo $row['harga_buku']; ?>">
                    <label for="exampleInputText1">Foto Buku</label>
                    <img src="img/foto buku/<?php echo $row['foto_buku']; ?>" width="65" height="100">
                    <input type="file" class="form-control" name="foto_buku">
                    <label for="exampleInputText1">Berat Buku</label>
                    <input type="number" class="form-control" placeholder="Berat Buku" name="berat_buku" value="<?php echo $row['berat_buku']; ?>">
                    <label for="exampleInputText1">Stok Buku</label>
                    <input type="number" class="form-control" placeholder="Harga Buku" name="stok_buku" value="<?php echo $row['stok_buku']; ?>">
                </div>
            </div>
            <!-- /.card-body -->
        <?php
    }
        ?>
        <div class="card-footer">
            <button type="submit" name="ubah" class="btn btn-primary">Submit</button>
            <button type="reset" class="btn btn-danger">Reset</button>
        </div>

        </form>
    </div>
    <!-- /.card --> error1.png 128.99 KBsaya ganti menjadi index.php?halaman=ubah_buku?id=<?= $data["id_buku"];
 ?>
error2.png 107.5 KBtidak tertampil ubah_barang.php