[bugfix] nullptr on no callback

srv/src/slack.js

@@ -52,7 +52,7 @@ function httpsRequest(url, cb) {
             try {
                 body = JSON.parse(body.toString("utf8"));
             } catch (e) {}
-            cb(res.statusCode, body);
+            cb && cb(res.statusCode, body);
         });
     });
 }