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

src/main/java/cvbuilder/view/MainViewer.java

@@ -5,10 +5,13 @@ import cvbuilder.model.UserGroup;
 
 import javax.swing.*;
 
-
 public class MainViewer {
 
+    private UserGroup userGroup;
+
     public MainViewer() {
+        userGroup = UserGroup.getInstance();
+
         JFrame viewer = new JFrame("CV Builder");
         viewer.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         viewer.setSize(800, 500);
@@ -65,13 +68,14 @@ public class MainViewer {
         ButtonGroup group = new ButtonGroup();
         String panelName = panel.getName();
 
-        for (User user : UserGroup.getUsers()) {
+        for (User user : userGroup.getUsers()) {
             JRadioButton radioButton = new JRadioButton(user.getUserAttribute(panelName));
             group.add(radioButton);
             panel.add(radioButton);
         }
     }
 
-
-
-}
+    public static void main(String[] args) {
+        SwingUtilities.invokeLater(MainViewer::new);
+    }
+}