DialogueHelper constructor Null safety

1. diginote
2. dialogue_helper
3. DialogueHelper
4. DialogueHelper constructor

DialogueHelper class

1. Constructors
2. DialogueHelper
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Static methods
12. cancelButton
13. okButton
14. saveButton
15. showConfirmationDialogue
16. showDestructiveDialogue
17. showErrorDialogue
18. showSuccessDialogue