the Changes are fixing a problem in the invoice creation

Web/app.py

@@ -7327,7 +7327,7 @@ def zurueckgeben(id):
 
     current_permissions = us.get_effective_permissions(session['username'])
         
-    if not item.get('Verfuegbar', True) and (current_permissions['actions'].get('can_manage_users', False)) or item.get('User') == username):
+    if not item.get('Verfuegbar', True) and (current_permissions['actions'].get('can_manage_users', False)) or item.get('User') == username:
         try:
             # Get ALL active borrowing records for this item and complete them
             client = MongoClient(MONGODB_HOST, MONGODB_PORT)
@@ -8623,10 +8623,27 @@ def admin_create_invoice(borrow_id):
         client = MongoClient(cfg.MONGODB_HOST, cfg.MONGODB_PORT)
         db = client[cfg.MONGODB_DB]
         student_cards = db['student_cards']
-        card = student_cards.find_one({'_id': ObjectId(borrow_id)})
+
+        student_id = borrow_doc.get('AusweisId') 
+        
+        if not student_id:
+            client.close()
+            flash('Dieser Ausleihe ist kein Schülerausweis zugeordnet.', 'error')
+            return redirect(url_for('admin_borrowings'))
+
+        # 2. Mit der korrekten ID in student_cards suchen
+        card = student_cards.find_one({'_id': ObjectId(student_id)})
+        
+        if not card:
+            client.close()
+            flash('Der zugehörige Schülerausweis wurde in der Datenbank nicht gefunden.', 'error')
+            return redirect(url_for('admin_borrowings'))
+
         card = _decrypt_student_card_doc(card)
         client.close()
-        borrower = card['SchülerName']
+        
+        # 3. Sicherer Zugriff mit .get() als zusätzliche Schutzmaßnahme
+        borrower = card.get('SchülerName', 'Unbekannt')
 
         invoice_number = existing_invoice.get('invoice_number') or _build_invoice_number(borrow_doc['_id'], now)
         item_name = item_doc.get('Name', '')