Please list the first names of the players with the most personal fouls in the 'NBL' league.
"NBL" is the lgID; most  personal foul refers to Max(Count(PF))
SELECT T1.firstName FROM players AS T1 INNER JOIN players_teams AS T2 ON T1.playerID = T2.playerID WHERE T2.lgID = 'NBL' GROUP BY T1.playerID, T1.firstName ORDER BY COUNT(PF) DESC LIMIT 1