This reverts commit 5fac6732:

  When the Divergence is solved and user goes back to the main view, the
  button is visible but when user clicks ont it and if Activities have
  been processed in the meantime, this will raise an AttributeError
  because newSolverProcess returns None and there is nothing to display.

  This happens even when adding Delivery_isDivergent() as a guard
  because it still returns True if Activities are running in background.