Handle the case where the base type of the lookup is QJSValue so that we
don't hit the assert.
However, it should not be possible to get a QJSValue there at all. This
should be investigated further. Created QTBUG-121662.
Pick-to: 6.7
Fixes: QTBUG-121393
Change-Id: I8bea87cbff74119bb977635ec391601f47269ff0
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Olivier De Cannière