In dataclasses.py line 218 there's the Source class with the contents method.
This method returns the self.line line from the self.parent.root.files.filepath(self.parent.root_module.id) file.

In my case, the line number is beyond the number of lines of the file and the handler crashes.

Upon inspection I found that the file in self.parent.root.files just contains a bunch of imports, but self refers to one of the imported files.
self.line and self.character really refer to the file in self.file_name, which is NOT the file retrieved from self.parent.root.files.

self.file_name is not in the list returned by self.parent.root.files.

Thanks!