fix for legacy validation

app/Controllers/Beheer/UsersController.php

@@ -75,7 +75,8 @@ class UsersController extends BaseController
 
         $user = $this->request->getPost();
 
-        $userID = $user['id'];
+        // Ensure we always have an ID, default to 0 for new user
+        $userID = (int)($user['id'] ?? 0);
         unset($user['id']);
 
         if ($userID == 0) {

app/Libraries/Validation/Validation.php

@@ -153,13 +153,41 @@ class Validation
     public function checkUnique(): void
     {
         $recordID = $this->request->getPost('id');
-        $model = $this->validationField['model'];
-        $field = $this->validationField['field'];
+        $model = $this->validationField['model'] ?? null;
+        $field = $this->validationField['field'] ?? null;
 
-        // For Eloquent models, we can call the static method directly
-        $value = $model::checkUnique($recordID, $field, $this->fieldValue);
+        if (!$model || !$field) {
+            return; // nothing to validate against
+        }
 
-        if (!$value) $this->errorMessages[] = 'invoer moet uniek zijn';
+        // Support legacy CI3 model naming like "User_model"
+        if (!class_exists($model) && str_ends_with($model, '_model')) {
+            $model = substr($model, 0, -6); // trim suffix
+        }
+
+        if (!class_exists($model)) {
+            // Fallback: cannot validate uniqueness without a model; do not hard-fail
+            return;
+        }
+
+        // Generic Eloquent uniqueness check
+        try {
+            $query = $model::where($field, '=', $this->fieldValue);
+
+            // ignore current record on update
+            if (!empty($recordID)) {
+                $query = $query->where('id', '!=', $recordID);
+            }
+
+            $exists = method_exists($query, 'exists') ? $query->exists() : ($query->count() > 0);
+
+            if ($exists) {
+                $this->errorMessages[] = 'invoer moet uniek zijn';
+            }
+        } catch (\Throwable $e) {
+            // Do not crash on validation; skip unique check if model/field invalid
+            return;
+        }
     }
 
     public function checkDate(): void