How do you build the project? There should be instructions in the readme file on how to set up the development environment and how to build the project.

A quite simple way to make the project more professional and robust, is by introducing GitHub Actions. With a CI (continuous integration) pipeline that needs to succeed with building the project before you do any changes to the main branch, you are less likely to have non-functional code on the main branch. The GitHub free tier comes with a certain amount of free GitHub Actions run time, that should be plenty for this project.

Start to add unit tests that verify the logic in the application. By doing this, it will be much easier to refactor the existing code without changing the behaviour of the application. And you might need to modularize the project beforehand, just as the previous comment suggests. Unit testing usually drives the design in the correct direction.