Fix crash of the QMessageBox when anothr instance of DQM Display is already running

=== dqm_display ===

Fix crash of the QMessageBox when DQM Display is already running (Serguei Kolos)
4 jobs for !255 with kolos/tdaq-09-04-00/dqm_display in 5 minutes and 11 seconds (queued for 3 seconds)
latest merge request