ENYO-354: Ensure we have a node when executing delayed measurements.

### Issue
A `moon.Header` can be destroyed soon after its creation. This can lead to the `asyncMethod` call attempting to make measurements on non-existent nodes.

### Fix
We guard against the case where we do not have an attached node. We might also want to investigate other cases where `asyncMethod` is used at create time for delayed work.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>