fix: populate missing letter_head_for in tabLetter Head and set default letterheads

erpnext/patches.txt

@@ -480,4 +480,5 @@ erpnext.patches.v16_0.merge_repost_settings_to_accounts_settings
 erpnext.patches.v16_0.set_root_type_in_account_categories
 erpnext.patches.v16_0.scr_inv_dimension
 erpnext.patches.v16_0.packed_item_inv_dimen
-erpnext.patches.v16_0.set_not_applicable_on_german_item_tax_templates
+erpnext.patches.v16_0.set_not_applicable_on_german_item_tax_templates
+erpnext.patches.v16_0.set_default_letter_head_for_doctype_and_report

erpnext/patches/v16_0/set_default_letter_head_for_doctype_and_report.py

@@ -0,0 +1,44 @@
+import frappe
+from frappe.query_builder import DocType
+
+
+def execute():
+	LH = DocType("Letter Head")
+	update_letter_head_for_query = (
+		frappe.qb.update(LH).set(LH.letter_head_for, "DocType").where(LH.letter_head_for.isnull())
+	)
+
+	update_letter_head_for_query.run()
+
+	for letter_head_for in ("DocType", "Report"):
+		default_exists = frappe.db.exists(
+			"Letter Head",
+			{
+				"is_default": 1,
+				"letter_head_for": letter_head_for,
+			},
+		)
+
+		if default_exists:
+			continue
+
+		standard_letter_head = frappe.db.get_value(
+			"Letter Head",
+			{
+				"standard": "Yes",
+				"disabled": 0,
+				"letter_head_for": letter_head_for,
+			},
+			"name",
+		)
+
+		if not standard_letter_head:
+			continue
+
+		frappe.db.set_value(
+			"Letter Head",
+			standard_letter_head,
+			"is_default",
+			1,
+			update_modified=False,
+		)