NEW HAVEN, Conn. — Kristine Nielsen, who is featured in the cast of Craig Lucas’ new play “The Singing Forest,” has temporarily left the Long Wharf Theater production because of a family emergency.She performed Wednesday, opening night of the work’s East Coast premiere, but will not appear in the show for the rest of the week. Understudy Sarah Peterson will play Nielsen’s role, the daughter of a famed lay analyst, in Lucas’ multigenerational, three-act play. The work received its world preem this summer at Seattle’s Intiman Theater. Lucas said he has dramatically revised the play since that run. Both productions are directed by Bartlett Sher. Nielsen is expected to return to the production next week and continue through the run, which ends Feb. 6.