fix: validate for active sla (#32132)

diff --git a/erpnext/support/doctype/service_level_agreement/service_level_agreement.py b/erpnext/support/doctype/service_level_agreement/service_level_agreement.py
index e49f212..7f4e9ef 100644
--- a/erpnext/support/doctype/service_level_agreement/service_level_agreement.py
+++ b/erpnext/support/doctype/service_level_agreement/service_level_agreement.py
@@ -776,6 +776,9 @@
 	if not parent.meta.has_field("service_level_agreement"):
 		return
 
+	if not parent.get("service_level_agreement"):
+		return
+
 	if (
 		doc.sent_or_received == "Received"  # a reply is received
 		and parent.get("status") == "Open"  # issue status is set as open from communication.py