You are having a nullpointer which indicates that something is empty what should have been present.
The exception occurs mail.service.connect, line 306, that is when the library tries to connect/authenticate against the smtp server.
Did you validate that your smtp authentication settings (username, password, etc) are not empty / null? Because if either of them are null then that would explain your nullpointer exception
And beware that when sending a html mail the plain text should also be filled otherwise you also can get an error like this. Not one where the cause is easily found.
Regards,
Ronald