Impossible to open pitivi installed via flatpak when I use a custom cursor theme

Hi

Both master and stable pitivi installed via flatpak are affected on Arch Linux running Gnome 3.28.2, tested these cursor themes:

• ArchCursorComplete
• Bibata_Amber
• Bibata_Oil
• Bibata_Ice
• BMZ
• Breeze_cursors
• Breeze_snow
• Bridge
• Paper

I get these messages in terminal:

Gdk-Message: Unable to load tcross from the cursor theme
Traceback (most recent call last):
  File "/app/bin/pitivi", line 168, in <module>
    _run_pitivi()
  File "/app/bin/pitivi", line 145, in _run_pitivi
    from pitivi import application
  File "/app/lib/pitivi/python/pitivi/application.py", line 33, in <module>
    from pitivi.mainwindow import MainWindow
  File "/app/lib/pitivi/python/pitivi/mainwindow.py", line 29, in <module>
    from pitivi.editorperspective import EditorPerspective
  File "/app/lib/pitivi/python/pitivi/editorperspective.py", line 42, in <module>
    from pitivi.timeline.timeline import TimelineContainer
  File "/app/lib/pitivi/python/pitivi/timeline/timeline.py", line 35, in <module>
    from pitivi.timeline.elements import Clip
  File "/app/lib/pitivi/python/pitivi/timeline/elements.py", line 31, in <module>
    from matplotlib.backends.backend_gtk3cairo import FigureCanvasGTK3Cairo as FigureCanvas
  File "/app/lib/python3.5/site-packages/matplotlib-2.0.2-py3.5-linux-x86_64.egg/matplotlib/backends/backend_gtk3cairo.py", line 6, in <module>
    from . import backend_gtk3
  File "/app/lib/python3.5/site-packages/matplotlib-2.0.2-py3.5-linux-x86_64.egg/matplotlib/backends/backend_gtk3.py", line 57, in <module>
    cursors.SELECT_REGION : Gdk.Cursor.new(Gdk.CursorType.TCROSS),
TypeError: constructor returned NULL