Crash when used with global menu under Plasma while editing new message

[Frank Fuhlbrück]

Start a KDE Plasma session and add the global menu widget (assuming all required packages are installed) and/or the HUD menu titlebar button for KWin , then start Thunderbird.
Initially the global menu is displayed as intended for the main window.
Try to write a new mail (Ctrl+N). When pressing Ctrl+N the message editor window briefly appears
and then Thunderbird crashes with SIGSEGV, see gdb.txt with debug symbols installed.
This is not specific to the hot key. Using the answer button etc. also results in the same crash.

1) Ubuntu version: Focal Fossa
2) Thunderbird version: 68.7.0+build1-0ubuntu1
3) Expected behavior: Message editor window appears
4) Actual behavior: Thunderbird crashes

I have tested the same procedure with the upstream binary release which does not contain the unity-menubar.patch (global menu etc. still enabled in Plasma). As expected, the menu is not exported (i.e., global menu is empty) and the message editor window opens regularly (with local menu). Furthermore, removing /usr/lib/x86_64-linux-gnu/libdbusmenu-glib.so.4 also breaks the global menu in the ubuntu build of thunderbird, but also prevents the crash, i.e., it behave like the upstream binary build. Thus the bug is mostly ubuntu specific (although other distros seem to apply the same patch).