I'm not going to claim this is the first regular cast member to die during their time as a regular on a show but there's at least one of significance that predates George Kearns. Veteran actor George Cleveland portrayed George "Gramps" Miller in "Lassie." Although the show was starting it's transition to the Martin family he died during the filming of the fourth season. "Gramps" death was written into the show. From a plot perspective his death prompted the Miller's to sell the farm to the recently moved into the area Martin family.