arb sometimes does not terminate (unreproducable)

[westram]

Under some situations arb fails to terminate properly, but that behavior is not reproducable.

Examined unreproducable situations:

1. Start arb with a w/o trees
   ⇒ warning window pops up ("no …").
   Now close main window.
   ⇒
   • normally warning window closes and arb terminates
   • sometimes warning window does not close and arb does not terminate
2. Start arb with a defect (e.g. "arb /proc/version") and press 'continue'
   Arb will display an error message.
   ⇒
   • normally arb closes after pressing ok
   • sometimes arb does not terminate

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see: #554, #436 (reproducable termination failures)