This seems not the whole code. But that’s my guess:
you called this.params._id; in a template-helper, not in the router-function as you showed in your code example.
FlowRouter.getParam("Id"); works everywhere and in particular in a template-helper, while this.params._id; can’t work in a helper, because this has a different meaning there.
If you need the appId in the “tasktemplate”, you should pass it as a param to this.render