In the Dialogue Editor's Actors section, inspect the "NPC" actor. Untick Custom Node Color, or make sure it's a visible color (e.g., 84, 84, 84, 255).
If that doesn't fix the issue, are there any errors or warnings in the Console window?
Does the same problem occur in a new, empty Unity 2019.1.2 project after importing the Dialogue System? Try inspecting one of the conversations in Plugins / Pixel Crushers / Dialogue System / Demo / Demo Database.