### Godot version

4.0 RC1

### System information

Linux Mint 20.3 Cinnamon

### Issue description

Tried converting a 3.5 game to 4.0 RC1. The conversion popup menu closed after a few seconds, and trying to edit the game crashes the entire engine with the following errors, which seem to be related to one music file not loading:
![errors](https://user-images.githubusercontent.com/125468288/219127087-f5a78035-4e4a-4625-aadb-e0cde6d5ac3d.png)

Not sure what's causing the issue, because when I created a new project with only that file and converted it I didn't get these errors, but consistently get them when trying to convert that game.

### Steps to reproduce

- Download the reproduction project below
- Import in Godot 4.0, and click "convert full project"
- If the bug happens, the engine will crash whenever trying to edit or run the converted project

### Minimal reproduction project

[Music Convertor Crash Demo.zip](https://github.com/godotengine/godot/files/10746591/Music.Convertor.Crash.Demo.zip)

if (dir->current_is_dir()) {
					directories_to_check.append(current_dir.path_join(file_name) + "/");
				} else {
					bool proper_extension = false;
					if (file_name.ends_with(".gd") || file_name.ends_with(".shader") || file_name.ends_with(".gdshader") || file_name.ends_with(".tscn") || file_name.ends_with(".tres") || file_name.ends_with(".godot") || file_name.ends_with(".cs") || file_name.ends_with(".csproj") || file_name.ends_with(".import"))
						proper_extension = true;
					if (proper_extension) {
						collected_files.append(current_dir.path_join(file_name));
					}
				}
				file_name = dir->_get_next();
			}
		} else {
			print_verbose("Failed to open " + path);
		}
	}
	return collected_files;
}
Vector<SourceLine> ProjectConverter3To4::split_lines(const String &text) {
	Vector<String> lines = text.split("\n");
	Vector<SourceLine> source_lines;
	for (String &line : lines) {
		SourceLine source_line;
		source_line.line = line;
		source_line.is_comment = false;
		source_lines.append(source_line);
	}
	return source_lines;
}
bool ProjectConverter3To4::test_conversion_gdscript_builtin(String name, String expected, void (ProjectConverter3To4::*func)(Vector<SourceLine> &, const RegExContainer &, bool), String what, const RegExContainer &reg_container, bool builtin_script) {