Attempt to assign property of non-object in Controller store method in laravel 5.2

Im new in laravel, please help me.. Im getting an error with

ErrorException in ShowuserController.php line 33: Attempt to assign property 'ipk' of non-object

I have a storage method in the controller and try to set properties like bellow

public function store(Request $request){
        $inputs = Input::all();
        $databeasiswa = new Databeasiswa;
        $databeasiswa->id = $inputs['id'];
        $databeasiswa = DB::table('beasiswas')
                        ->join('databeasiswas', 'beasiswas.id', '=', 'databeasiswas.beasiswa_id')
                        ->get();
        $databeasiswa = DB::table('mahasiswas')
                        ->join('databeasiswas', 'mahasiswas.id', '=', 'databeasiswas.mahasiswa_id')
                        ->get();
        $databeasiswa->ipk = $inputs['ipk'];
        $databeasiswa->kelas = $inputs['kelas'];
        $databeasiswa->prestasi = $inputs['prestasi'];
        $databeasiswa->detail_prestasi = $inputs['detail_prestasi'];
        $databeasiswa->organisasi = $inputs['organisasi'];
        $databeasiswa->jabatan = $inputs['jabatan'];
        $databeasiswa->save(); 
        return redirect(url('databeasiswa'));
    }

And I also using combobox in my view like bellow

<form action="" method="post" accept-charset="utf-8" role="form"> 
  <div class="box-body">
      <div class="form-group">
        <label>NIM Mahasiswa</label>
        <input type="text" name="id" class="form-control" placeholder="">
      </div>

      <div class="form-group">
        <label>IPK</label>
        <select name=ipk class="form-control">
        <option value="">Pilih IPK</option>
        <option value="5">> 3,80</option>    
        <option value="4">3,61 - 3,80</option>    
        <option value="3">3,41 - 3,60</option>    
        <option value="2">3,21 - 3,40</option>
        <option value="1">3,00 - 3,20</option>
        </select>
      </div>

via Chebli Mohamed