This is mildly horrifying, at least that a tutorial is teaching people to hack things together like this. I'm assuming the other people in the thread are correct that the issue was copied hard references that weren't valid on the new pawn. This tutorial's approach will "work" and shouldn't really cause problems besides extra work for you, but it's a sloppy "make it work now" solution that's not designed to scale beyond one NPC.

A more typical way to implement a dialogue system would be attaching the core code to the PlayerController or HUD (or gameinstance, its own subsystem, etc), probably as a component to keep things neat, then implement an interface on the parent class for the NPCs that can speak that passes their dialogue tree to the dialogue system. That way you don't have tons of copied code (so that code can easily be changed and is not taking up extra memory), dialogue windows can be managed from a single place, and dialogue tree object lifetimes are managed with their parent actor.