OSX uiwriter stacktrace needed

Hi,

so before pushing the unocrsrptr branch, I tried to use our CI:

 https://gerrit.libreoffice.org/#/c/16179/

however it turned out the uiwriter unittest was unstable when basing on it
today: I had crashes on Linux too, but to make it more fun not under gdb. With
no way to verify my changes any further I pushed the branch.

However, OSX seems to be reliably broken now according to the tinderboxes.
If someone manages to provide a backtrace, I would appreciate that, so I can
investigate what broke and fix it.

Best,

Bjoern