It is not possible to supply custom data to the tube map about sequencetubemap

[wolfib]

The "custom"-button is mostly only relevant if you are running your own instance of sequenceTubeMap. In this case you can supply your own data and pick among the files in your data folder by selecting "custom".
The "Reload"-button triggers another look at this data folder and updates the drop down menu options, so that you can pick from all the files which are currently in your data folder.
In the online demo page, there is currently only a single xg file to pick after selecting "custom". If you are not selecting any file in the "xg file" dropdown, you'll encounter the error you posted about.

[wolfib]

You can now select "custom (file upload)" as a data source option, which allows uploading and displaying your own local files as long as they are small enough.