qt's QFileDialog's setDefaultSuffix bug

I created a new class QmayaFileDialog inheriting from QFileDialog , the in showEvent()function , I call function setDefaultSuffix , then when I open this dialog to save file , when I type the name of the folder existing , it should help me select this folder and the button becomes "open" ,then I press enter to go into the folder , and also if you type
or // in the nameLineEdit , then continue to enter text , it will block there , becoming very slow because this is network path , it is looking the path when you enter each text , this is very annoyed . But if I didn't call this function ,then the problem disappeared .