fix: 'NoneType' object is not iterable (backport #39977) (#39980)

fix: 'NoneType' object is not iterable (#39977)

(cherry picked from commit 8e7d47b3a7)

Co-authored-by: rohitwaghchaure <rohitw1991@gmail.com>

erpnext/stock/doctype/pick_list/pick_list.py

@@ -32,6 +32,10 @@ class PickList(Document):
 		self.update_status()
 		self.set_item_locations()
 
+		if self.get("locations"):
+			self.validate_sales_order_percentage()
+
+	def validate_sales_order_percentage(self):
 		# set percentage picked in SO
 		for location in self.get("locations"):
 			if (