// old code
if (ConfigReader. disable_worlds. isEmpty ( ) ) {
    List <String > disable_worlds = ConfigReader. disable_worlds ;
    for ( String w : disable_worlds ) if (player. getWorld ( ). getName ( ). equals (w ) ) return ;
}
// new code
if ( !ConfigReader. disable_worlds. isEmpty ( ) ) {
    List <String > disable_worlds = ConfigReader. disable_worlds ;
    for ( String w : disable_worlds ) if (player. getWorld ( ). getName ( ). equals (w ) ) return ;
}