April's ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack.

Some spells and abilities that create Mutagen tokens require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. The Mutagen token won't be created.

Mutagen tokens are a kind of predefined token. Each one is a colorless artifact with the artifact subtype Mutagen and the ability ", , Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery."