at that moment if the military/fema took over, technically it wouldnt be trump doing it.

http://law2.umkc.edu/faculty/PROJECTS/FTRIALS/CONLAW/inaugurationconstit.html

Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:--"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States."

seems pretty clear that there will be a small amount of time where there is no president