I had exacly the same problem on arch linux with the same versions of QT and QTcreator.
I included on the file ".qmlproject" the line:
importPaths: ["/usr/lib/qt/qml/"]
And it worked. The red underline disapeared and can use design now.
Try to find something like "/qt/qml/" on your installation folder and include it on the importPaths
.