Hi @user123 (great name! 
)
I just tried it with Thunderbird 65.0b2 and had the same issue.
The PDFCreator Log shows the following:
Log Output
2019-01-16 11:32:36.9268 [Info] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Conversion.Actions.Actions.EMailClientAction.ProcessJob: Launched client e-mail action
2019-01-16 11:32:36.9376 [Info] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Conversion.Actions.Actions.EMailClientAction.Process: Launched client e-mail action
2019-01-16 11:32:36.9376 [Info] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Conversion.Actions.Actions.EMailClientAction.Process: Starting e-mail client
2019-01-16 11:32:36.9600 [Info] 2824-10 (ProcessingThread) pdfforge.Mail.MapiClient.ShowEmailClient: Launched client email action
2019-01-16 11:32:36.9737 [Info] 2824-10 (ProcessingThread) pdfforge.Mail.MapiClient.ShowEmailClient: Start MAPI processing
2019-01-16 11:32:36.9737 [Info] 2824-10 (ProcessingThread) pdfforge.Mail.MapiMailMessage.ShowDialogInOwnThread: Creating MAPI thread
2019-01-16 11:32:36.9737 [Info] 2824-14 () pdfforge.Mail.MapiMailMessage._ShowMail: Showing mail client
2019-01-16 11:32:36.9737 [Info] 2824-14 () pdfforge.Mail.MapiMailMessage._ShowMail: Subject:
2019-01-16 11:32:36.9737 [Info] 2824-14 () pdfforge.Mail.MapiMailMessage._ShowMail: Adding 1 files
2019-01-16 11:32:36.9835 [Info] 2824-14 () pdfforge.Mail.MapiMailMessage._ShowMail: Starting MAPI call
2019-01-16 11:32:36.9835 [Info] 2824-10 (ProcessingThread) pdfforge.Mail.MapiClient.ShowEmailClient: Done with MAPI
2019-01-16 11:32:36.9835 [Info] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Conversion.Actions.Actions.EMailClientAction.Process: Done starting e-mail client
2019-01-16 11:32:36.9835 [Trace] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Core.Workflow.JobRunner.CallActions: Action EMailClientAction completed
2019-01-16 11:32:36.9835 [Debug] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Core.Workflow.JobRunner.CleanUp: Cleaning up after the job
2019-01-16 11:32:36.9835 [Trace] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Core.Workflow.JobRunner.RunJob: Job finished successfully
2019-01-16 11:32:36.9991 [Trace] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Core.Workflow.JobRunner.RunJob: Calling job completed event
2019-01-16 11:32:37.0362 [Debug] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Utilities.Threading.ThreadManager.StartSynchronizedThread: Adding thread
2019-01-16 11:32:37.0938 [Trace] 2824-10 (ProcessingThread) pdfforge.PDFCreator.Core.Workflow.JobInfoQueueManager.ProcessJobs: Removing job from Queue
2019-01-16 11:32:42.1885 [Info] 2824-14 () pdfforge.Mail.MapiMailMessage._ShowMail: MAPI result was: -2147023170
2019-01-16 11:32:42.1885 [Info] 2824-14 () pdfforge.Mail.MapiMailMessage._ShowMail: Done with MAPI
Basically, the log tells us that PDFCreator starts Thunderbird using MAPI.
Since Thunderbird for Windows 64-Bit is still an unstable beta, you should consider reporting the issue to mozilla if they're not already aware of the issue.
As a workaround you can use the script action to start Thunderbird. Should look like this:
(Should work with the 64-Bit version, too. I don't have an account to test it on my virtual machine but the software didn't crash on start-up.)
You can use the command line arguments for Thunderbird. Find them here: Command line arguments - Thunderbird - MozillaZine Knowledge Base