channelUrl property Null safety

1. sendbird_sdk
2. base_message
3. BaseMessage
4. channelUrl property

BaseMessage class

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