Fix assert with Meta modifier

Meta modifier is not found in some configurations which triggers an
assert. Instead of assert, ignore the modifier if it is not found.

Task-number: QTBUG-52298
Change-Id: I258cee4014a30162afebb423eadaf07ef0ed6a1f
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>