QtCreator does not suggest available signals and slots in connect statement

Using Qtcreator 4.8.2 and Qt 5.12.1 and 5.12.2 in archlinux/ubuntu, I'm trying to connect a signal to a slot using traditional method of QObject::connect, but when I open bracket after SIGNAL or SLOT macro, code completion does not suggest me available signals or slots. I haven't seen such a problem since I have updated Qt to this latest version. What's the problem?

example of the code:
 connect(serial, SIGNAL(readyRead()),this, SLOT(setEncValue()));
readyRead() and setEncValue() are not available in suggestion list, but not only these two slot and signal, there are actually no signals or slots shown in the suggestion list.