Why does someone say "it's safe to break an ashtray"?

It is to relieve the anxiety caused by breaking the ashtray.

In our habits, breaking things is considered a bad omen, especially on festive days. So everyone will handle things carefully to prevent them from being broken. But if it is really accidentally broken, people generally say "breaking the peace", thus eliminating or offsetting the anxiety caused by breaking things. What makes individuals feel broken does not mean bad, and there is no need to feel uneasy and anxious about it.