ApplicationWindow/SplitView is not a type

When running "app" from https://github.com/mitchcurtis/slate/tree/qt-6 with https://codereview.qt-project.org/c/qt/qtquickcontrols2/+/307865/ checked out (and https://codereview.qt-project.org/c/qt/qtquickcontrols2/+/308053 cherry-picked on top, if it's not already merged to dev), I get

qrc:/qml/main.qml:32:1: ApplicationWindow is not a type

and occasionally

qrc:/qml/main.qml:187:5: SplitView is not a type

when running it.

The output when running with QML_IMPORT_TRACE is attached; one log for each error.