Error using https://github.com/Maatwebsite/Laravel-Excel Call to a member function has() on null

I am using https://github.com/Maatwebsite/Laravel-Excel package to import some data from excel. I am using the ToCollection concern with following code

The controller method is:

public function import(Request $request)
    {
        $this->validate($request, [
            'excel' => 'required|mimes:xls,xlsx,csv',
        ]);

        Excel::import(new StudentsImport, request()->file('excel'));
    }

The StudentImport class:

class StudentsImport implements ToCollection, WithCalculatedFormulas
{
    public function collection(Collection $rows)
    {
        $english = Subject::find(1);
        $nepali = Subject::find(2);

        foreach ($rows as $row) {  
            Student::create([
                'roll_no' => $row[0],
                'full_name' => $row[1],
                'sex' => $row[6],
                'father_name' => $row[7],
                'mother_name' => $row[8],
                'address' => $row[9],
                'total_marks_obtained' => $row[87],
                'total_percentage' => $row[88],
                'grade_points' => $row[90],
                'rank' => $row[91],
            ]);
        }
    }
}

The loop is workking fine and the code is getting out of the loop, however no Student is being created and i am getting this error: Call to a member function has() on null

PhpOffice\PhpSpreadsheet\Worksheet\Worksheet::cellExists
C:\laragon\www\marksheet\vendor\phpoffice\phpspreadsheet\src\PhpSpreadsheet\Worksheet\Worksheet.php:1340