[Crash] Tympan crashes after managing 'Messages' window

User story :
As Tympan user, I want Code_TYMPAN not to crash when managing 'Messages' window, in order not to lose my work.

Steps to reproduce crash :

• Launch Tympan
• Move 'Messages' window on 'Gestion sites' window
  • 'Messages' window disappears
  • KO => we shoud have a window with 2 tabs.
• Press button 'Fenêtre Messages' to show it
  • Tympan crashes.

Acceptance criteria :

• 2 tabs appear when placing a window on top of another,
• no crash when clicking on button 'Fenêtre Messages'