fix error response for web APIs

ingress/plugin/grpc.go

@@ -82,6 +82,10 @@ func (p *grpcPlugin) SetRule(ctx context.Context, rule *ingress.Rule, opts ...in
 }
 
 func (p *grpcPlugin) Close() error {
+	if p.conn == nil {
+		return nil
+	}
+
 	if closer, ok := p.conn.(io.Closer); ok {
 		return closer.Close()
 	}