fix: erase default before adding new series-es (backport #18080) (#18081)

* fix: erase default before adding new series-es

(cherry picked from commit f5a45a7e78)

* chore: conflict

Co-authored-by: Ankush Menat <ankush@frappe.io>

frappe/core/doctype/document_naming_settings/document_naming_settings.py

@@ -113,6 +113,8 @@ class DocumentNamingSettings(Document):
 
 		option_string = "\n".join(options)
 
+		# Erase default first, it might not be in new options.
+		self.update_naming_series_property_setter(doctype, "default", "")
 		self.update_naming_series_property_setter(doctype, "options", option_string)
 		self.update_naming_series_property_setter(doctype, "default", default)