Mysterious "Attempted to evaluate an expression in an invalid context"

This was seen with qt/qt-qml HEAD at 6170493cac58b30635c5037341bbcb059ee6d193
and tesla HEAD at 9d58e3566ff2cd376ffea06cd1d3fffbd37c2e9c

Start tesla, then press ctrl+r to reload it.
"QDeclarativeExpression: Attempted to evaluate an expression in an invalid context" is
printed in the Application Output window, with no indication of what caused it.

(scroll through the pages of icons in the launcher and more of these are printed on reloading
the app, so it's probably related to this JS function call
onDestroyed: DropSupport._garbageCollect();
in common/dragndrop/DragAndDropArea.qml)