Webhook settings api tests

netbox_sys_plugin/api/serializers.py

 from rest_framework import serializers
 from virtualization.models import VirtualMachine, Cluster, ClusterType
-from ipam.models import Service
 from netbox.api.fields import ChoiceField
 from dcim.api.nested_serializers import (
     NestedDeviceSerializer, NestedDeviceRoleSerializer, NestedPlatformSerializer, NestedSiteSerializer,
@@ -207,6 +206,12 @@ class WebhookSettingsSerializer(NetBoxModelSerializer):
         model = WebhookSettings
         fields = '__all__'
 
+    def validate(self, data):
+        # Validate uniqueness
+        if not self.partial and WebhookSettings.objects.exists():
+            raise serializers.ValidationError("You can only have one webbook setting.")
+        return data
+
 class ProviderTypeExtraConfigSerializer(NetBoxModelSerializer):
     id = serializers.IntegerField(read_only=True)
     extra_config_structure = serializers.JSONField()