private void CreateSubjectButton_Click(object sender, RoutedEventArgs e)
 {
     if (SubjectName.Text != "")
     {
         bool isCorrect = true;
         foreach (var n in subjects)
         {
             if (SubjectName.Text == n.SubjectName)
             {
                 isCorrect = false;
             }
         }
         if (isCorrect)
         {
             string subjectName = SubjectName.Text;
             string answer      = client.AddSubject(subjectName);
             if (answer == "succesfull")
             {
                 main.Content = new AdminSubjects(client, mw, main);
             }
         }
         else
         {
             MessageBox.Show("This item already exists!");
         }
     }
     else
     {
         MessageBox.Show("Please enter subject name!");
     }
 }