Thanks for the reply, I tried your suggestion but got the following error: passing 'const QString' as 'this' argument discards qualifiers [-fpermissive]

This is the code I wrote for it
read.h
Qt Code: 
  1. #ifndef READ_H
  2. #define READ_H
  3.  
  4. #include <QDialog>
  5.  
  6.  
  7. namespace Ui {
  8. class read;
  9. }
  10.  
  11. class read : public QDialog
  12. {
  13.     Q_OBJECT
  14.  
  15. friend class beastiary;
  16.  
  17. public:
  18.      const QString nvalue;
  19.      void ulk();
  20.      explicit read(QWidget *parent = 0);
  21.        ~read();
  22.  
  23. private:
  24.      Ui::read *ui;
  25.  
  26. };
  27.  
  28.  
  29. #endif // READ_H
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beastiary.cpp
Qt Code: 
  1. void beastiary::on_listWidget_itemClicked(QListWidgetItem *item)
  2. {
  3.    const QString nvalue  = ui->listWidget->currentItem()->text();
  4.    read red;
  5.    red.nvalue=nvalue;
  6.    red.setModal(true);
  7.    red.exec();
  8.    red.ulk();
  9.  
  10. }
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read.cpp
Qt Code: 
  1. void read::ulk()
  2. {
  3.     QFile file (nvalue);
  4.     if (!file.open(QFile::ReadOnly | QFile::Text))
  5.      {
  6.          QMessageBox::warning (this, "Error", "File Could not Be Opened");
  7.      }
  8.   QString line;
  9.   QTextStream in(&file);
  10.   while (!in.atEnd())
  11.    {
  12.     line = in.readLine();
  13.     ui->lineEdit_name_r->setText(ui->lineEdit_name_r->text()+line);
  14.     qDebug() <<line;
  15.    }
  16. }
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