fix the use of 'length()' for strings
fix the use of 'length()' for strings

tsuijten/kotlin-web-site,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,tsuijten/kotlin-web-site,hltj/kotlin-web-site-cn,tsuijten/kotlin-web-site,tsuijten/kotlin-web-site