fixed issue so that UserGroup stays as a singleton instance so that all users are stored in it properly

src/main/java/cvbuilder/model/UserGroup.java

@@ -4,14 +4,21 @@ import java.util.ArrayList;
 
 public class UserGroup {
 
-    private static ArrayList<User> users;
+    private static UserGroup instance;
+    private ArrayList<User> users;
 
     private UserGroup() {
         users = new ArrayList<>();
     }
 
+    public static UserGroup getInstance() {
+        if (instance == null) {
+            instance = new UserGroup();
+        }
+        return instance;
+    }
 
-    public static ArrayList<User> getUsers() {
+    public ArrayList<User> getUsers() {
         return users;
     }