You have undefined behavior somewhere else in your program -- my guess would be that you actually casted that pointer into a std::istream, even though it is not. We would need to see where in is instantiated before being able to run down exactly what's going on. Look for any casts to istream in your codebase and I bet you find the problem.
This probably indicates that your is member does not point to a stringstream. It might just be initialized to some garbage value when enclosing object is instantiated.
If you are testing for pointer being zero, make sure it's set to zero in the constructor (and reset to zero if you ever detach the stream).
You've answered the question in the last two lines. You have a dangling pointer to an object that's gone out of scope, and using it will cause undefined behaviour (which could well be a crash). You need to make sure the object exists for as long as it's needed, by placing it in a longer-lasting scope or allocating it with new and making sure it is deleted when no longer needed.