message: Fix crash on extraction exception
  Regression introduced in the `improved-progress-info` branch.
diff: @@ -279,6 +279,9 @@ bool PerformUpdateFromSourcesWithUI(wxWindow *parent,
  return false;
  }
  + if (!pot)
  + return false;
  +
  bool cancelledByUser = false;
  if (skipSummary || ShowMergeSummary(parent, catalog, pot, &cancelledByUser))
  {
  