Not 100% certain, but aren't you asking for a Link to an item that might not exist yet? I'm assuming you're using this as a form for SharePoint list creation and have just clicked "new" to add a new entry (that you might not have submitted yet).
I've made a smilar test but not met the same problem like your description.
Here's my test:
You can access the properties for the SharePointIntegration control only when the form is running in SharePoint, not when you're customizing the form in PowerApps Studio. These properties may not be available in OnStart or OnVisible.
What's more, have you connect the form with outlook?