This warning is benign. You could find a discussion of the same issue in this post:
From the code you shared, it seems that the dialogueCallback is declared but not assigned to DialogueEventCallback. Without this assignment, the callback won’t execute as expected.
To resolve this, please assign the callback in the Start or Awake method like this:
dialogueCallback = DialogueEventCallback;
Hope this helps, let me know if the issue persist.