QtGui5 not loaded - c++

Question

I am developing an application in C++, winApi and Qt. What this application does is that it communicates through serial COM ports.
A user can open multiple pairs of ports and an opened port listens other port constantly.
To implement this functionality I use windows threads.There is a method named startRead() that reads from other port constantly and changes the text area.
void SendAndReceive::startRead(){
DWORD numRead = 0;
std::string hex;
while (1)
{
char *buffer = (char *)malloc(sizeof(char) * 500);
BOOL ret = ReadFile(portHandler, buffer, 500, &numRead, 0);
if (!ret)
{
std::string errorMessage = "";
}
buffer[numRead] = '\0';
std::string receivedData(buffer);
QString QData(receivedData.c_str());
if (ui->checkBox->isChecked())
{
std::string receivedData(buffer);
hex= stringToHex(receivedData);
QString QData1(hex.c_str());
emit asHex(QData1);
}
QString QData2(receivedData.c_str()) ;
emit finished(QData2);
free(buffer);
}
}
And there is another thread that writes data periodically to other port. For example , if you enter 2 seconds in a text line , the program writes data to other port at every 2 seconds and this method is writePeriodic().
void SendAndReceive::writePeriodic(){
DWORD numWritten;
while (1 && checkWrite == true )
{
Concurrency::wait(timePeriod*1000);
QString QData = ui->textEdit->toPlainText();
std::string data = QData.toStdString();
WriteFile(portHandler, data.c_str(), strlen(data.c_str()), &numWritten, NULL);
}
}
So when I run this program , program runs smoothly for 2 or 3 minutes and then it crashes and I get errors like "Program has stopped working" , "program closed unexpectedly".When I debug program , it says something like "Qt5Core not loaded" or "Qt5Gui not loaded".
Before I ask here , I did some search on the web . First I did not use emit finished(QString) signal , instead I directly manipulated GUI objects inside startRead() method.(I did something like ui->text_edit->setText(some QString)).But after some search I learn that I can not change GUI objects from another thread so I decided to use signal and slot mechanism and it has not solved my problem so far.I get same error again and again.Please tell me what I am doing wrong . If you need further explanation, I will happily give more details.