Problem when my system closes suddenly with Ubuntu 18.04 and QT Creator and C++

Question

I´m developing a Ubuntu application with QT Creator in c++ language and i have a problem when my system shutdowns suddenly. I´m constantly modifying some files and when my system shutdowns doesn´t save the last changes but it add several "0" chars at the last line. I always make flush and close the file but anyway i have the problem.

Anybody can help me?

This is my code:

AddItemLog(unsigned char Index,QString Cadena)
{
    QFile File(FileNames[Index]);
    File.open(QIODevice::ReadWrite | QIODevice::Append);
    QTextStream Stream(&File);

    Stream << GetStringDate(false) << ": " << Cadena.toUtf8() << endl;

    File.flush();
    File.close();
}