Is employment a requirement for HSMP Tier 1 extension?

Is it a requirement for HSMP Tier 1 General extension that applicant must have a job?
What happens if a person get unemployed/lose her job at the time of applying for new extension, provided it meets all other requirements?

There is no requirement to be employed when extending a Tier 1 visa. As long as you meet the points criteria, other circumstances don't matter.