Here’s a note for anyone having trouble installing the Smart App.
I paired a couple of sensors with my ST hub, and they were reporting data, so I knew they’re working. I also created a PlantLink account, but did not add a hub as I have no intention of using it. But, when I tried to install the Smart App, at the point where you select the sensors I did that, but tapping on Done generated the red bar of death and “An unexpected error occurred.” I was unable to complete the install of the Smart App.
The problem turned out to be the device type. For some reason they paired using the “PlantLink Direct Sensor” type and not the official “PlantLink” type. Once I changed that in the IDE I was able to get the Smart App to install.