Is Brexit at risk if Boris Johnson succumbs?

Johnson was taken to the hospital with COVID-19 issues.

Suppose the worst happens. As I understand British government, the Conservative Party would nominate a new prime minister who, if elected by his party, would serve.

But Labor (and other parties) could have a vote of No Confidence and bring down the government (again), in which case there would be full blown elections again.

So hypothetically, if Johnson isn’t there, could this series of events reopen the entire Brexit question?

(Yes, I know that Johnson signed the document a few months ago, but nothing serious goes into effect until the end of the year, and I doubt there’s much negotiating taking place these days.)

Could COVID-19 bring down the British government and reopen Brexit?