I just got the confusing error message:
No object of type org.apache.tapestry5.services.Heartbeat is available from the Environment.
That happened because Heartbeat had been pushed and popped during the render cycle. The problem is, that Heartbeat is still a key in the map of available objects, but the list/stack of associated objects is empty.
The patch removes entries removes those keys from the AvailableValues values list.