Hmm, that line should set current.mixAlpha. Pharan! :angry:
For from.mixingFrom to be non-null and from.mixDuration to be 0, you'd have to be setting a new current animation without having applied the previous one. We have some code to discard animations that were set, not applied, and then replaced, so I'm not really sure how you got in that state.
Still, it doesn't hurt to avoid setting a NaN (mmmm naan) mixAlpha. If mixDuration is 0 then the mix is 1 in this code: float alpha = from.alpha * entry.mixAlpha * (1 - mix), which means mixAlpha gets multiplied by zero so its value doesn't matter. That makes the fix:
if (from.mixingFrom != null && from.mixDuration > 0)
current.mixAlpha *= Math.min(from.mixTime / from.mixDuration, 1);
