Nullptr access in CommonSettingsChanged() (KICAD-39)

Sentry Issue: KICAD-39

EXCEPTION_ACCESS_VIOLATION_READ / 0x50: Fatal Error: EXCEPTION_ACCESS_VIOLATION_READ / 0x50
  File "kiway.cpp", line 595, in KIWAY::CommonSettingsChanged
    frame->CommonSettingsChanged( aEnvVarsChanged, aTextVarsChanged );
  File "pcb_edit_frame.cpp", line 1161, in PCB_EDIT_FRAME::ShowBoardSetupDialog
    Kiway().CommonSettingsChanged( false, true );
  File "dialog_drc.cpp", line 817, in DIALOG_DRC::OnEditViolationSeverities
    m_frame->ShowBoardSetupDialog( _( "Violation Severity" ) );
  File "kicad.cpp", line 453, in APP_KICAD::OnRun
    return wxApp::OnRun();
  File "exe_common.inl", line 102, in invoke_main
...
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