chore(aci): handle OrganizationMember.DoesNotExist when getting target (#102113)

If there is no org member associated with this user and organization,
then we should exit gracefully.

Author: mifu67

src/sentry/incidents/endpoints/serializers/alert_rule_trigger_action.py

@@ -32,6 +32,8 @@ def human_desc(
                 return "Send a notification to " + target.get_email()
             elif target_type == AlertRuleTriggerAction.TargetType.TEAM.value:
                 return "Send an email to members of #" + target.slug
+        else:
+            return "Send a notification to [removed]"
     elif action_type == AlertRuleTriggerAction.Type.OPSGENIE.value:
         if priority:
             return f"Send a {priority} Opsgenie notification to {target_display}"

src/sentry/notifications/notification_action/group_type_notification_registry/handlers/metric_alert_registry_handler.py

@@ -46,10 +46,14 @@ def target(action: Action) -> OrganizationMember | Team | str | None:
         target_type = action.config.get("target_type")
         if target_type == ActionTarget.USER.value:
             dcga = DataConditionGroupAction.objects.get(action=action)
-            return OrganizationMember.objects.get(
-                user_id=int(target_identifier),
-                organization=dcga.condition_group.organization,
-            )
+            try:
+                return OrganizationMember.objects.get(
+                    user_id=int(target_identifier),
+                    organization=dcga.condition_group.organization,
+                )
+            except OrganizationMember.DoesNotExist:
+                # user is no longer a member of the organization
+                pass
         elif target_type == ActionTarget.TEAM.value:
             try:
                 return Team.objects.get(id=int(target_identifier))