When compiling Qt Creator in debug, the integrated .ui Designer crashes at initialization (e.g. when opening a .ui file).

objc[20830]: Class QMacAutoReleasePoolTracker is implemented in both ***/QtOnline/5.12.0/clang_64/lib/QtCore.framework/Versions/5/QtCore_debug (0x10fc5fc20) and ***/QtOnline/5.12.0/clang_64/lib/QtCore.framework/Versions/5/QtCore (0x136b78068). One of the two will be used. Which one is undefined.
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QPixmap: Must construct a QGuiApplication before a QPixmap

It looks like some code path in Qt Designer loads release libraries which in turn pull in release Qt frameworks, which leads to duplicate symbols.

This is probably related to the change https://codereview.qt-project.org/222788 in Qt 5.12 that applications now directly link against the debug version of frameworks when built in debug.