Use of Animator with invalid target prevents Qt Quick animations in the following tests from start

If QQuickWindow in animatorInvalidTargetCrash() test (quick/qquickanimations/tst_qquickanimations.cpp) is not hidden before it is deleted, animations in the following tests won't work. The bug occurs only for threaded render loop.
Related change: https://codereview.qt-project.org/#/c/180386/7/tests/auto/quick/qquickanimations/tst_qquickanimations.cpp