Yes you can leave Apple Crisp out at room temperature overnight.
Apple Crisp does not need refrigeration and can be left out overnight without any issues.
You can leave Apple Crisp out at room temperature without refrigeration for up to 4 days before it goes bad.
If you plan to keep the Apple Crisp longer than 4 days then it should be refrigerated and if you plan to keep the Apple Crisp longer than that you should freeze it instead.
When frozen the Apple Crisp will keep for up to 6 months.
However the longer you keep the Apple Crisp frozen the less fresh it becomes and it will change in taste and quality.
It may remain safe to eat after 6 months but it won't be as good.