ges_clip_get_top_effect_index fails sometimes

ges_clip_get_top_effect_index fails (returns -1) when called after ges_clip_set_top_effect_index if the clip has a lot of effects (~10).

PoC: yatinmaan/gst-editing-services@48c01e3d

I can clean up the Test/PoC and open an MR if its useful.