fix: add nil check for Writer in FlushWriter function

relay/helper/common.go

@@ -15,6 +15,9 @@ import (
 )
 
 func FlushWriter(c *gin.Context) error {
+	if c.Writer == nil {
+		return nil
+	}
 	if flusher, ok := c.Writer.(http.Flusher); ok {
 		flusher.Flush()
 		return nil
@@ -28,14 +31,14 @@ func SetEventStreamHeaders(c *gin.Context) {
 		return
 	}
 
+	// 设置标志，表示头部已经设置过
+	c.Set("event_stream_headers_set", true)
+
 	c.Writer.Header().Set("Content-Type", "text/event-stream")
 	c.Writer.Header().Set("Cache-Control", "no-cache")
 	c.Writer.Header().Set("Connection", "keep-alive")
 	c.Writer.Header().Set("Transfer-Encoding", "chunked")
 	c.Writer.Header().Set("X-Accel-Buffering", "no")
-
-	// 设置标志，表示头部已经设置过
-	c.Set("event_stream_headers_set", true)
 }
 
 func ClaudeData(c *gin.Context, resp dto.ClaudeResponse) error {