Not sure if I'm doing something wrong, but in line 337 of Dialogue.gd, next() is being called if I press ui_accept, ui_select, or continue_dialogue even if there is no dialogue (for example, clicking a button on my main menu), resulting in the game crashing. This can be reproduced using the example project and hitting the enter key before doing anything else. I'm currently working around it by returning out of next() if current_id == null, but I'm not sure if that will lead to other problems.
EDIT: Looks like it was fixed in the most recent build. I'll leave the topic up in case somebody else is using an older version and comes across the same issue; feel free to delete it at your discretion.