Fixed missing non terminal error

Cargo.toml

@@ -16,7 +16,7 @@ rand = "~0.9"
 thiserror = "~2.0"
 
 [dev-dependencies]
-divan = "0.1.21"
+divan = "~0.1"
 
 [[bench]]
 name = "e2e"

src/mcts.rs

@@ -75,9 +75,16 @@ impl<'conf, S: GameState + std::fmt::Debug> MCTS<'conf, S> {
         if !selected_node.state.is_terminal() {
             self.expand(selected_id);
             let children: &Vec<usize> = &self.arena.get_node(selected_id).children;
-            let random_child: usize = *children.choose(&mut rand::rng()).unwrap();
+            match children.choose(&mut rand::rng()) {
+                Some(&random_child) => {
                     selected_id = random_child;
                 }
+                None => {
+                    // We ran out of nodes
+                    return Err(MCTSError::NonTerminalGame);
+                }
+            }
+        }
         let rewards = self.simulate(selected_id);
         self.backprop(selected_id, &rewards);