I deleted the calendar by mistake. But I can't reinstall it from the App Center. The package id's '' are not valid' was introduced when an error occurred during installation.
Here on elementaryos.stackexchange.com is just a community of users with more or less experience in using elementary OS. Probably some developpers also read the questions, but the official way to report bugs like this has to be done into the elementary OS devs team on gitHub - https://github.com/elementary.