Upgrade from CiviCRM 5.12.4 to 5.13.5 generates Sparkpost PHP fatal error
See if https://github.com/cividesk/com.cividesk.email.sparkpost/pull/77 fixes your problem.  It's been merged but there has not been a release of the sparkpost extension since then so either pull from git or apply the fix manually - it's only one line of code.