Accessing QFontComboBox causes SIGSEGV if wrong/non existing font had been set with setCurrentFont()

Problem: QFontComboBox causes a segmentation fault if a non-existent font had been set with setCurrentFont().

Steps to reporoduce:

1. Create object QFont tableFont
2. Set the object to a non-existing font, e.g. by restoring settings with QSettings that does not provide a default value. 
3. Create object QFontComboBox  comboFont
4. Set said font object to  QFontComboBox by using comboFont.setCurrentFont(tableFont)
5. Watch Qt producing the following error:
   QFont::fromString: Invalid description ',-1,-1,5,400,0,0,0,0,0,0,0,0,0,0,1'
6. Access comboFont. , e.g. by setting {{comboFont.}}setFontFilters(QFontComboBox::ProportionalFonts)
7. Segmentation fault occurs on both Windows and Linux. 

See attached backtraces for the GUI thread.