Mail crashes upon launch

Suddenly, Mail crashes upon launch continually. OS Sierra;10.12.6

I get the following message:

Application Specific Information:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'

abort() called

terminating with uncaught exception of type NSException

Any help gratefully appreciated.