On choosing Add to Calendar the app may crash. This issue arose with the release of iOS 12 and appears only to affect users running that version.
The crash appears to be caused by a bug in the operating system, and is discussed in further detail here: https://forums.developer.apple.com/thread/109374
(Note: the workaround mentioned in the forum thread is not one that we're able to use right now, as it requires the 'Add to Calendar' function to be accessed differently.)
We're continuing to look for solutions to this problem and will endeavour to deploy a fix or workaround as soon as possible. We apologise for any inconvenience.
UPDATE, 16 Jan 2019: this issue is fixed in v4.5.1 - please update.