[minor][buying] fix schedule date check
diff --git a/erpnext/controllers/buying_controller.py b/erpnext/controllers/buying_controller.py
index f00f6cc..8ab5128 100644
--- a/erpnext/controllers/buying_controller.py
+++ b/erpnext/controllers/buying_controller.py
@@ -430,7 +430,8 @@
 				if not d.schedule_date:
 					d.schedule_date = self.schedule_date
 
-				if d.schedule_date and getdate(d.schedule_date) < getdate(self.transaction_date):
+				if (d.schedule_date and self.transaction_date and
+					getdate(d.schedule_date) < getdate(self.transaction_date)):
 					frappe.throw(_("Row #{0}: Reqd by Date cannot be before Transaction Date").format(d.idx))
 		else:
 			frappe.throw(_("Please enter Reqd by Date"))