I get the same error when click on some "talk to"  button on demo:

E 0:00:41.560   get_node: Node not found: MSG_pos.
  <C++ Error>   Condition "!node" is true. Returned: __null
  <C++ Source>  scene/main/node.cpp:1381 @ get_node()
  <Stack Trace> MSG_Parser.gd:266 @ update_text()
                MSG_Parser.gd:164 @ next()
                MSG_Parser.gd:253 @ _auto_next()
                MSG_Parser.gd:198 @ next()
                MSG_Parser.gd:253 @ _auto_next()
                MSG_Parser.gd:103 @ start_dialogue()
                Character.gd:13 @ talk()
                talk_button.gd:9 @ on_pressed()

at 206 line of MSG_Parser.gd:
if current_speaker.get_node("MSG_pos"):

Hi again,
Error occurs because there are not attached Position2d nodes (MSG_pos) to characters and player in demo.
That resolves this issue.

Did you manage to resolve this? I am getting the same MSG_pos errors, but I think it's because the current_speaker var is null.

Any chance you could do a tutorial video, or a write up of how to implement your system? The documentation doesn't explain a lot of details.