Return function returns null!

Hello, I would like to ask.
I am creating my minigame plugin, and I created class called "Arena". Then I created class "ArenaManager", which was managing Arenas (such as createArena, removeArena, getArena). Then I created a listener, which contains OnPlayerInteractEvent. (If you click a sign, - arenaManager.getArena(signLine1))
But my ArenaManager getArena() returns null. Why? Please help. I spent 3 days on this mistake and i still havent solved it yet.
Codes:
ArenaManager getArena() code:

public Arena getArena(String arenaName) {
for(Arena arena : arenas) {
if(arena.name.equalsIgnoreCase(arenaName)) {
return arena;
}
}
return null;
}

JoinSignListener code:

Arena arena = Tunnels.arenaManager.getArena(arenaName);

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After this, Arena arena = null. WHY?