Gnome Shell should terminate running Calculator operations when closing overview

In the Activities search field, if I type an expression that can be evaluated by the Calculator search plugin, the calculation keeps running even after I close Activities. This becomes a problem when the expression is extremely hard to calculate but easy to type: a good example is "10!!!". This causes the gnome-calculator process to keep running in the background and consuming 100% CPU.

Steps to reproduce:

1. Enable the Calculator search plugin.
2. Open Activities.
3. Type "10!!!" in the search field. Calculator now attempts to calculate a ridiculously big factorial.
4. Close activities.
5. Notice your laptop fan is suddenly whirring for no apparent reason.
6. Examine running processes to find gnome-calculator using 100% of a single core, because the factorial would take ages to compute.

This could potentially be a Calculator bug, too. Feel free to reassign it as necessary.