1.2.3 Try it and follow the instructions for the version above.
OK, try again in 1.2.4. I fixed it. If you have any other issues, please report them. I haven't tested this, as it may be specific to your project or other plugins.
So, feel free to report any issues.
Open the Plugin Manager and ensure it loads after Core/VisuMZ (if applicable).
Clear the cache (Ctrl+F5) and run the test on the map with the balloon.
After some experiment, I think I figured when this error occurs. After setting a balloon on an NPC, if you you later clear the balloon using "clear NPC balloon" command, save the game and exit, and then reload the save and try to enter the map where the NPC balloon was triggered, it gives you an error.
Edit: to be more precise, entering the map where the NPC with the balloon icon on top of it that was cleared by "Clear balloon" command, gives an error.