iOS: Fix error when adding attendees due to UUID is nil.

[GoldenSoju]

I recently had several crashes on my app (iOS users) and found out that the adding attendee functionality was broken.

Error:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSDictionaryM setObject:forKeyedSubscript:]: key cannot be nil'

I was able to locate the error in the SwiftDeviceCalendarPlugin.swift file (line 692):

ekEvent!.setValue(attendees, forKey: "attendees")

And adding the line below to the createParticipant in the same file solved the problem. Thanks to this solution.

participant.setValue(UUID().uuidString, forKey: "UUID")

Besides that I

• had to change some null errors in the example app (/event_attendee.dart),
• changed a color value for dark mode in (/calendar_event.dart) as the greenAccent[100] looks awful in dark mode. ;-),
• updated rrule package dependency from 0.2.7 to 0.2.10,
• deleted unneccessary dependency in _test/device_calendartest.dart

[IVLIVS-III]

Looks good on first glance, thanks. Will hopefully have time to do a proper review in the coming days.