pinnedMessagesUpdatedAt property Null safety

1. sendbird_sdk
2. group_channel
3. GroupChannel
4. pinnedMessagesUpdatedAt property

GroupChannel class

1. Constructors
2. GroupChannel
3. fromJson
4. fromJsonAndCached
5. Properties
6. accessCodeRequired
7. channelType
8. channelUrl
9. coverUrl
10. createdAt
11. creator
12. customType
13. data
14. dirty
15. fromCache
16. hashCode
17. hiddenState
18. invitedAt
19. inviter
20. isBroadcast
21. isDiscoverable
22. isDistinct
23. isEphemeral
24. isExclusive
25. isFrozen
26. isHidden
27. isPublic
28. isStrict
29. isSuper
30. joinedAt
31. joinedMemberCount
32. key
33. lastMessage
34. lastPinnedMessage
35. memberCount
36. members
37. messageOffsetTimestamp
38. messageSurvivalSeconds
39. myCountPreference
40. myLastRead
41. myMemberState
42. myMutedState
43. myPushTriggerOption
44. myRole
45. name
46. pinnedMessageIds
47. pinnedMessagesUpdatedAt
48. primaryKey
49. runtimeType
50. unreadMentionCount
51. unreadMessageCount
52. Methods
53. copyWith
54. deleteChannel
55. getMember
56. getPollChangeLogsSinceTimestamp
57. getPollChangeLogsSinceToken
58. noSuchMethod
59. pinMessage
60. toJson
61. toString
62. unpinMessage
63. updateChannel
64. Operators
65. operator ==
66. Static methods
67. createChannel
68. getChannel
69. refresh