Hi,
Sorry to hear you are having problems getting widgets working. Crash logs for the preview app will show up in ~/Library/Logs/DiagnosticReports/. It can sometimes take a few minutes for it to generate, so give it a bit time and then see if any are there. Hopefully that will give you some helpful hints on what is causing the crash.
To start from a fresh slate you can use the terminal app and run
xcrun simctl --set previews delete all.
If that still doesn't resolve issue then probably best next step will be to file a feedback with diagnostics.
We will need the diagnostics Xcode Previews generates in order to make sure we understand the error the previews system is encountering.
When you get an error in Xcode Previews, an error banner appears in the canvas
Click the "Diagnostics" button in that banner
In the sheet that appears, click "Generate Report" in the bottom left of the sheet
Attach (or make from the folder) the resulting zip file to the bug (will be named something like previews-diagnostics-0123456789.zip)
