Fix for filterString not being on main thread
Fix for filterString not being on main thread

Simple fix for filterString not being called on the main thread.
Found using XCode thread sanitisation.

ViennaRSS/vienna-rss,josh64x2/vienna-rss,lapcat/vienna-rss,barijaona/vienna-rss,lapcat/vienna-rss,josh64x2/vienna-rss,josh64x2/vienna-rss,lapcat/vienna-rss,ViennaRSS/vienna-rss,barijaona/vienna-rss,ViennaRSS/vienna-rss,Eitot/vienna-rss,barijaona/vienna-rss,josh64x2/vienna-rss,josh64x2/vienna-rss,Eitot/vienna-rss,Eitot/vienna-rss,barijaona/vienna-rss,Eitot/vienna-rss,ViennaRSS/vienna-rss,barijaona/vienna-rss,ViennaRSS/vienna-rss,lapcat/vienna-rss