GTK: app segfaults if unable to connect to display

[mhsmith]

Describe the bug

This appeared in #1687 when xvfb-run was missing from the Toga testbed CI command. I can also reproduce it locally.

Steps to reproduce

Unset the DISPLAY environment variable, then try to run the app:

$ briefcase dev

[testbed] Starting in dev mode...
===========================================================================
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused

(__main__.py:20685): Gtk-CRITICAL **: 08:59:26.455: _gtk_style_provider_private_get_settings: assertion 'GTK_IS_STYLE_PROVIDER_PRIVATE (provider)' failed

(__main__.py:20685): Gtk-CRITICAL **: 08:59:26.460: _gtk_style_provider_private_get_settings: assertion 'GTK_IS_STYLE_PROVIDER_PRIVATE (provider)' failed

(__main__.py:20685): Gtk-CRITICAL **: 08:59:26.460: _gtk_style_provider_private_get_settings: assertion 'GTK_IS_STYLE_PROVIDER_PRIVATE (provider)' failed
Fatal Python error: Segmentation fault

Current thread 0x00007ff9f3404740 (most recent call first):
  File "/home/smith/git/beeware/toga/gtk/src/toga_gtk/icons.py", line 14 in __init__
  File "/home/smith/git/beeware/toga/core/src/toga/icons.py", line 77 in __init__
  File "/home/smith/git/beeware/toga/core/src/toga/app.py", line 455 in icon
  File "/home/smith/git/beeware/toga/core/src/toga/app.py", line 330 in __init__
  File "/home/smith/git/beeware/toga/testbed/src/testbed/app.py", line 27 in main
  File "/home/smith/git/beeware/toga/testbed/src/testbed/__main__.py", line 4 in <module>
  File "/home/smith/.pyenv/versions/3.8.12/lib/python3.8/runpy.py", line 87 in _run_code
  File "/home/smith/.pyenv/versions/3.8.12/lib/python3.8/runpy.py", line 97 in _run_module_code
  File "/home/smith/.pyenv/versions/3.8.12/lib/python3.8/runpy.py", line 207 in run_module
  File "<string>", line 1 in <module>

Problem running application 'testbed'

AppImage is much worse because it doesn't give any hint of what the cause of the crash is:

$ briefcase run

[testbed] Starting app...
===========================================================================
/tmp/.mount_Toga_TpKBh1e/AppRun.wrapped: line 11: 22376 Segmentation fault      "${APPDIR}/usr/bin/python3" -u -s -X utf8 -c "import runpy, sys; sys.path.pop(0); runpy.run_module('${BRIEFCASE_MAIN_MODULE}', run_name='__main__', alter_sys=True)" "$@"

Problem running app testbed.

Expected behavior

When unable to connect to the display, both run and dev should give an actionable error message, not a segfault, and ideally not a Python traceback either.

Screenshots

No response

Environment

• Operating System: Debian 10
• Python version: 3.8
• Software versions:
  • Briefcase: 0.3.11
  • Toga: 0.3.0dev39

Logs

No response

Additional context

No response

[freakboy3742]

Fixed by #1921