After upgrading my Pixel 3 XL to the beta of Android 12, I can no longer open messages to view them in K-9 Mail. When I tap on a message in the message list to open it, the app crashes. I’ve joined the beta program and I’m running the most recent beta, same problem.
The K-9 Mail app developer explained on Github (thanks!) that this is a known issue with Android SystemWebView. The most recent version in the Play Store is currently incompatible with Android 12. The fix is to uninstall the updates to the WebView app, re-enable the app after doing so, and turn off automated updates to the app until Android 12 is released to the world, by which point the version in the Play Store will presumably be fixed.