English parser crashes with sensible entry

For example an even with title "Holiday from 20th to 21st" crashes the calendar (due to a missing null check at https://github.com/elementary/calendar/blob/44ad95c504e199ef8bc4887758702c2ff2f3e0f2/src/EventParser/Parsers/ParserEn.vala#L449)

(A lot of seemingly sensible constructions are ignored).

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/55176452-english-parser-crashes-with-sensible-entry?utm_campaign=plugin&utm_content=tracker%2F60388692&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F60388692&utm_medium=issues&utm_source=github).