fix: fetch from more than one sales order in Maintenance Visit (#26924)

* [fix] #26336

* fix(ux): make customer field reqd for fetching SO

Co-authored-by: Ankush Menat <ankush@iwebnotes.com>
diff --git a/erpnext/maintenance/doctype/maintenance_visit/maintenance_visit.js b/erpnext/maintenance/doctype/maintenance_visit/maintenance_visit.js
index 53ecdf5..6b3f184 100644
--- a/erpnext/maintenance/doctype/maintenance_visit/maintenance_visit.js
+++ b/erpnext/maintenance/doctype/maintenance_visit/maintenance_visit.js
@@ -73,12 +73,16 @@
 		if (this.frm.doc.docstatus === 0) {
 			this.frm.add_custom_button(__('Maintenance Schedule'),
 				function () {
+					if (!me.frm.doc.customer) {
+						frappe.msgprint(__('Please select Customer first'));
+						return;
+					}
 					erpnext.utils.map_current_doc({
 						method: "erpnext.maintenance.doctype.maintenance_schedule.maintenance_schedule.make_maintenance_visit",
 						source_doctype: "Maintenance Schedule",
 						target: me.frm,
 						setters: {
-							customer: me.frm.doc.customer || undefined,
+							customer: me.frm.doc.customer,
 						},
 						get_query_filters: {
 							docstatus: 1,
@@ -104,12 +108,16 @@
 				}, __("Get Items From"));
 			this.frm.add_custom_button(__('Sales Order'),
 				function () {
+					if (!me.frm.doc.customer) {
+						frappe.msgprint(__('Please select Customer first'));
+						return;
+					}
 					erpnext.utils.map_current_doc({
 						method: "erpnext.selling.doctype.sales_order.sales_order.make_maintenance_visit",
 						source_doctype: "Sales Order",
 						target: me.frm,
 						setters: {
-							customer: me.frm.doc.customer || undefined,
+							customer: me.frm.doc.customer,
 						},
 						get_query_filters: {
 							docstatus: 1,
diff --git a/erpnext/selling/doctype/sales_order/sales_order.py b/erpnext/selling/doctype/sales_order/sales_order.py
index bba5401..5d44582 100755
--- a/erpnext/selling/doctype/sales_order/sales_order.py
+++ b/erpnext/selling/doctype/sales_order/sales_order.py
@@ -718,8 +718,7 @@
 				"doctype": "Maintenance Schedule Item",
 				"field_map": {
 					"parent": "sales_order"
-				},
-				"add_if_empty": True
+				}
 			}
 		}, target_doc)
 
@@ -745,8 +744,7 @@
 				"field_map": {
 					"parent": "prevdoc_docname",
 					"parenttype": "prevdoc_doctype"
-				},
-				"add_if_empty": True
+				}
 			}
 		}, target_doc)