iOS build crashes on tapping on Chat again after closing Chat window

[ann0see]

Describe the bug

Building Jamulus for iOS on Qt6 with setWindowState ( Qt::WindowMaximized ); set in the constructor of CChatDlg provides a "close" button and a somewhat usable UI for the chat window.
However, after closing the window and then tapping on the Chat button on the main window again crashes the app with a NULL pointer dereference (?)
ASSERT failure in QCoreApplication::sendEvent: "Unexpected null receiver", file /Users/qt/work/qt/qtbase/src/corelib/kernel/qcoreapplication.cpp, line 1577
To Reproduce

• Build Jamulus with Qt6 for iOS,
• Open the app
• Tap on Chat
• Close chat by tapping on the close button
• Tap on Chat again and observe the app crash

Expected behavior
Tapping on Chat again doesn't crash the application.

Screenshots

Operating system
iOS 17

Version of Jamulus

3.10.0dev-58000016
Additional context
I believe that we leak a pointer somewhere/don't recreate it. By calling close() it gets deallocated and then we get a null pointer somewhere.

[ann0see]

I believe this doesn't occur with Qt5 - but I can't reliably test this.

[ann0see]

Fixed by #3413