Currently, from CKEditor menu toolbar (XWiki 13.10) or quick actions on XWiki 15.x user can decide to print all list of tasks using Tasks macro. But, if the user submits empty input, the error message will be displayed with ids null.
GIF (from 15.8 XWiki)

To prevent this, when selecting Tasks macro, the user must provide at least one task id because this parameter is mandatory.