Fix crash when the operator search is used while no active object exists. The cause of the issue is an attempt to dereference `ob` when it is `NULL`. Therefore this patch checks the return value of `SCULPT_mode_poll()` first, to ensure that `ob` isn't `NULL`. Reviewed By: pablodp606 Maniphest Tasks: T74838 Differential Revision: https://developer.blender.org/D7156