add_schedule_popup library Null safety

1. diginote
2. add_schedule_popup library

diginote package

1. Libraries
2. add_message_popup
3. add_schedule_popup
4. add_screen_popup
5. add_template_popup
6. background_colour_selector
7. colour_picker_icon
8. date_time_selector
9. dialogue_helper
10. firebase_login_provider
11. firebase_login_repository
12. firebase_options
13. firebase_preview_provider
14. firebase_preview_repository
15. firebase_register_provider
16. firebase_register_repository
17. firebase_screen_info_provider
18. firebase_screen_info_repository
19. firebase_screens_provider
20. firebase_screens_repository
21. firebase_templates_provider
22. firebase_templates_repository
23. font_picker
24. font_selector
25. foreground_colour_selector
26. generated_plugin_registrant
27. header_footer
28. header_input
29. home_view
30. icon_helper
31. input_validators
32. login_view
33. main
34. message_input
35. message_item
36. message_item_content
37. message_item_paneled
38. message_size_selector
39. messages_model
40. navigation_page_enum
41. notification_service
42. online_status_icon
43. preview_view
44. register_view
45. screen_info_model
46. screen_item
47. screen_item_content
48. screen_model
49. screen_settings_popup
50. screens_view
51. settings_view
52. shared_preferences_theme_manager
53. state_enums
54. styling_constants
55. templates_model
56. templates_view
57. templates_view_popup
58. text_alignment_selector
59. theme_provider
60. timer_provider
61. token_updater_service
62. zoom_provider