fix: harden custom QR login output

client.go

@@ -10,6 +10,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"reflect"
 	"strconv"
 	"strings"
 	"sync"
@@ -39,7 +40,15 @@ func WithHTTPClient(httpClient *http.Client) Option {
 // WithQRWriter configures where QR login output is written.
 func WithQRWriter(writer io.Writer) Option {
 	return func(client *Client) error {
-		if writer == nil {
+		isNil := writer == nil
+		if !isNil {
+			value := reflect.ValueOf(writer)
+			switch value.Kind() {
+			case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
+				isNil = value.IsNil()
+			}
+		}
+		if isNil {
 			return fmt.Errorf("QR writer must not be nil")
 		}
 		client.qrWriter = writer