Call to a member function subjects() on null

i'm facing an error Call to a member function subjects() on null . My project is by php and laravel. My project is on live server. Its works fine normally but when i click to edit something or click update button then it shows Call to a member function subjects() on null . Only my current browser facing this error but if i hit from another browser to my server its seems ok.

public function index()

{

    $featured_packages = Package::where('featured_package', 1)

        ->with('reviews', 'user')

        ->get();

    $popular_packages = Package::where('popular_package', 1)

        ->with(['class' => function ($query) {

            $query->pluck('name');

        }, 'reviews', 'user'])

        ->withCount('mcqs')->get();

    $class_id  = StudentClass::all()->random()->id;

    if (Auth::check()) {

        $class_id = auth()->user()->grad != '' ? auth()->user()->grad : $class_id;

    }

    $subjects = StudentClass::find($class_id)->subjects()->withCount('packages')->get();

    return view('frontend.index', compact('featured_packages', 'popular_packages', 'subjects', 'class_id'));

}

public function about()

{

    $about = About::orderBy('id', 'desc')->first();

    return view('frontend.about', compact('about'));

}

public function contact()

{

    return view('frontend.contact');

}

public function faqs()

{

    $faqs = Faq::orderBy('id', 'asc')->where('status', 1)->get()->groupBy('tab');

    return view('frontend.faqs', compact('faqs'));

via Chebli Mohamed