src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java

@@ -143,7 +143,6 @@ class ExpenseListController {
     }
 
     @PostMapping("/accept-invite")
-
     public ResponseEntity<?> acceptInvite(@RequestBody InviteRequest inviteRequest) {
         ExpenseList list = expenseListService.findByInviteCode(inviteRequest.getInviteCode());
 
@@ -151,6 +150,9 @@ class ExpenseListController {
                 list.getInviteCodeExpiration().isBefore(LocalDateTime.now())) {
             return ResponseEntity.status(HttpStatus.NOT_FOUND).body("Invalid or expired invite code");
         }
+        if (list.getSharedWith() != null) {
+            return ResponseEntity.status(HttpStatus.IM_USED).body("List has already been shared");
+        }
         AppUser user = null;
         try {
             user = userService.getUser(inviteRequest.getUserId());

src/main/java/de/zendric/app/xpensely_server/services/ExpenseListService.java

@@ -71,7 +71,7 @@ public class ExpenseListService {
             if (expenseList.getOwner().getId().equals(id)) {
                 userSpecificList.add(expenseList);
             } else {
-                if (sharedWith.getId().equals(id)) {
+                if (sharedWith != null && sharedWith.getId().equals(id)) {
                     userSpecificList.add(expenseList);
                 }
             }
@@ -88,7 +88,7 @@ public class ExpenseListService {
             if (expenseList.getOwner().getUsername().equals(username)) {
                 userSpecificList.add(expenseList);
             } else {
-                if (sharedWith.getUsername().equals(username)) {
+                if (sharedWith != null && sharedWith.getUsername().equals(username)) {
                     userSpecificList.add(expenseList);
                 }
             }