Sometimes the Rector crashes on exception and we need more detailed output to fix it or report a bug.
You can use
--debug option, that will print nested exceptions output:
vendor/bin/rector process src/Controller --debug
Also an Xdebug:
--xdebug option when running Rector
vendor/bin/rector process src/Controller --dry-run --xdebug
The best way to report a bug is then find the smallest possible
rector.php config and single file in your project that causes the crash. Do not copy-paste the whole file, it would be miss leading. The crash usually happens on a 5-10 lines of code.
When you identify those, use https://getrector.com/demo to create a reproducer.
Then you can create an an issue or even a tests. Just click on the button on the right under the code.