src/AiPlayer.java

@@ -155,6 +155,7 @@ public abstract class AiPlayer extends LocalPlayer implements Runnable {
         while(i.hasNext()) {
             Thread thread = i.next();
             try {
+                thread.interrupt();
                 thread.join();
                 i.remove();
             } catch (InterruptedException e) {

src/MainFrame.java

@@ -129,7 +129,7 @@ public class MainFrame extends JFrame {
      * @author Lucas Bronson, Peer Ole Wachtel, Luca Conte
      */
     public void showPanelWin(String panelName, Player player){
-        if(player != gameBoard.getP1()){
+        if(gameBoard == null || player != gameBoard.getP1()){
             return;
         }
 
@@ -152,7 +152,7 @@ public class MainFrame extends JFrame {
      * @author Lucas Bronson, Peer Ole Wachtel, Luca Conte
      */
     public void showPanelLose(String panelName, Player player){
-        if(player != gameBoard.getP1()){
+        if(gameBoard == null || player != gameBoard.getP1()){
             return;
         }