scheduledMessageId property Null safety

1. sendbird_sdk
2. scheduled_user_message
3. ScheduledUserMessage
4. scheduledMessageId property

ScheduledUserMessage class

1. Constructors
2. ScheduledUserMessage
3. fromJson
4. Properties
5. apnsBundleId
6. appleCriticalAlertOptions
7. channelType
8. channelUrl
9. createdAt
10. customType
11. data
12. dedupId
13. errorCode
14. extendedMessage
15. forceUpdateLastMessage
16. hashCode
17. isOperatorMessage
18. isPinnedMessage
19. isSilent
20. markAsRead
21. mentionedUserIds
22. mentionedUsers
23. mentionType
24. message
25. messageId
26. messageSurvivalSeconds
27. metaArrays
28. ogMetaData
29. parentMessage
30. parentMessageId
31. parentMessageText
32. reactions
33. replyToChannel
34. requestedMentionUserIds
35. requestId
36. runtimeType
37. scheduledAt
38. scheduledInfo
39. scheduledMessageId
40. scheduledStatus
41. sender
42. senderId
43. sendingStatus
44. sendPush
45. sentAt
46. threadInfo
47. type
48. updatedAt
49. Methods
50. applyReactionEvent
51. applyThreadInfoUpdateEvent
52. getMetaArrays
53. getThreadedMessagesByTimestamp
54. isResendable
55. noSuchMethod
56. toJson
57. toString
58. Operators
59. operator ==