Edit: Figured it out.
I have now completely deleted my Yahoo account rather than disable it. I set it up as a new account and everything seems to be working fine now.
outofexile
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Figure it out. I disabled all my accounts in Settings > Internet Accounts. It was then possible to launch Mail. Then I added each account and quit then relaunched Mail. I did this until I got to my Yahoo! account. This was the account that was crashing mail. I tried to rebuild the Yahoo! on and it did not work, so I've left that one disabled. Otherwise, it's working fine now.
Excellent simple fix. Thanks @therunway12
Same here. Hoping for a workaround also.