Question: An election for the leadership of the Ulster Unionist Party (UUP) was held on 8 April 2017 at the party's Annual General Meeting, the 2017 contested election was triggered after incumbent Leader Mike Nesbitt, elected in 2012, announced following the 2017 assembly election his intention to step down as party leader, the 2017 election to the Northern Ireland Assembly was held on which date?
Sentence (from "Northern Ireland Assembly election, 2017"): McGuinness' position was not filled, and thus by law his resignation triggered an election.
Relevant evidence: No