John Gill Commentary Acts 19:39

"But if ye seek anything about other matters, it shall be settled in the regular assembly." — Acts 19:39 (ASV)

But if you inquire anything concerning other matters
Than what belongs to the craft and business of Demetrius, and the artificers:

it shall be determined in a lawful assembly ;
that is, called together according to law, and who have a right to hear, try, and judge causes, which such a confused lawless assembly as this in the theatre had not.