fix missing reference for Act object destructor

[TheBitFighter]

When using smart pointer objects, the compiler expects a call to the destructor, but there is no destructor implementation in source. This fixes the issue by using the default destructor for the time being.

[rmanohar]

I'd fix this slightly differently: have an empty destructor with a placeholder comment that says that we need to implement it properly and clean up all memory. If you'd like to do this, let me know; otherwise I can do this and push a fix. It's also okay to just use the default and put a comment next to it indicating that this needs to be fixed properly down the road.