Cubic 2019.03-47 does not launch on Lubuntu 18.10

Cubic 2019.03-47 does not launch on Lubuntu 18.10, with the following error:

Traceback (most recent call last):
  File "/usr/share/cubic/cubic.py", line 31, in <module>
    import display
  File "/usr/share/cubic/display.py", line 35, in <module>
    gi.require_version('GtkSource', '3.0')
  File "/usr/lib/python3/dist-packages/gi/__init__.py", line 127, in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace GtkSource not available