fix: Use .set method to set serial no.
diff --git a/erpnext/stock/doctype/pick_ticket/pick_ticket.py b/erpnext/stock/doctype/pick_ticket/pick_ticket.py
index eb54730..18382e6 100644
--- a/erpnext/stock/doctype/pick_ticket/pick_ticket.py
+++ b/erpnext/stock/doctype/pick_ticket/pick_ticket.py
@@ -61,7 +61,7 @@
 		'item_code': item.item,
 		'warehouse': item.warehouse
 	}, limit=item.qty, order_by='purchase_date')
-	item.serial_no = '\n'.join([serial_no.name for serial_no in serial_nos])
+	item.set('serial_no', '\n'.join([serial_no.name for serial_no in serial_nos]))
 
 def set_batch_no(item, doc):
 	batches = frappe.get_all('Stock Ledger Entry',
diff --git a/erpnext/stock/doctype/pick_ticket_item/pick_ticket_item.json b/erpnext/stock/doctype/pick_ticket_item/pick_ticket_item.json
index 6db3284..7095be6 100644
--- a/erpnext/stock/doctype/pick_ticket_item/pick_ticket_item.json
+++ b/erpnext/stock/doctype/pick_ticket_item/pick_ticket_item.json
@@ -72,7 +72,6 @@
    "read_only": 1
   },
   {
-   "depends_on": "has_serial_no",
    "fieldname": "serial_no",
    "fieldtype": "Small Text",
    "label": "Serial No",
@@ -130,7 +129,7 @@
   }
  ],
  "istable": 1,
- "modified": "2019-07-25 11:18:58.478250",
+ "modified": "2019-07-26 14:47:33.965373",
  "modified_by": "Administrator",
  "module": "Stock",
  "name": "Pick Ticket Item",