Exception after .net 6 upgrade

Question

The following code (which worked fine in .net core 3.1)

var jobType = AppDomain.CurrentDomain
.GetAssemblies()
.SelectMany(x => x.DefinedTypes)
.Single(x => x.FullName == className);

is throwing the exception after the upgrade to .net 6

className is the namespace to a class we need to dynamically instantiate my.redacted.namespace.Jobs.ReminderMail.MonthlyUpdateReminderJob

Is this way of dynamically getting the Type not possible anymore and what would be the way to go?

Thanks in advance