Yes, I have the save! In fact, I loaded into a save when I was still under the Southern Empire, waited until I found an army, then saved. I clicked join army and it crashed again, so the issue is still here and can be found in this save file!
If this save works, you should be brought to the dialogue with the army leader. Just clicking 'Join Army' should do it!
It is now uploaded, save file named 'Rhagaea Dead', description 'Rhagaea Dead Southern Empire Join Army Crash'. Hope it helps!