perf: Add exception handling to increase robust code

src/main/java/edu/group5/app/model/user/Customer.java

@@ -31,19 +31,25 @@ public Customer(int userId,
                   String email,
                   String passwordHash) {
 
-      super(userId, "Customer", firstName, lastName, email, passwordHash);
-      this.preferences = new ArrayList<>();
+    super(userId, "Customer", firstName, lastName, email, passwordHash);
+    this.preferences = new ArrayList<>();
   }
 
-    public List<Integer> getPreferences() {
-        return preferences;
-    }
+  public List<Integer> getPreferences() {
+    return preferences;
+  }
 
-    public void addPreference(int orgNumber) {
-        preferences.add(orgNumber);
+  public void addPreference(int orgNumber) {
+    if (!preferences.contains(orgNumber)) {
+      throw new IllegalArgumentException("Organization number already in preferences");
     }
+    preferences.add(orgNumber);
+  }
 
-    public void removePreference(int orgNumber) {
-        preferences.remove(Integer.valueOf(orgNumber));
+  public void removePreference(int orgNumber) {
+    if (!preferences.contains(orgNumber)) {
+      throw new IllegalArgumentException("Organization number not found in preferences");
     }
+    preferences.remove(Integer.valueOf(orgNumber));
+  }
 }