Catch 'NoneType' errors

(Feel free to just update file manually. I usually create feature branches when sending pull requests but didn't this time)

When playing guild wars 2 I'd occasionally get an error about 'NoneType' object not iterable when calling `getAverageScreenColor()`.  I haven't been able to reliably reproduce it so instead I "fix" it by logging the exception as warning so it keeps running.  There may be a better fix for it but I didn't spend too much looking in to it and this fixes the symptoms at least.