Missing safety checks for guessWindowXID()

[marcohu]

guessWindowXID() can return null, but the return value is not always safe-guarded which can potentially lead to errors as some code expects it to be non-null.

[marcohu]

Test case:

• disable Pixel Saver
• Reload GNOME-Shell (Alt+F2 r)
• Start Meld
• Create two files pal.txt and pal1.txt
• Compare these two files with Meld
• Start an additional Meld instance (to have two application windows, the 2nd window must appear above the first one, that is key)
• Enable Pixel Saver
  -> Warnings appear in the log
• Disable Pixel Saver
  -> Error