fix github bug #8: fpGUI as x11 root window fails.
fix github bug #8: fpGUI as x11 root window fails.

Solution gave by Mohaslan. Checking if AWindow or AProperty actually
have valid values resolves the problem.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI