effects/private: Fix crash when shutting down desktop grid and overview

Because of const/non-const issues, iterator got invalidated in the loop,
which led to a crash.

Cases that trigger it: interaction with a thumbnail while the effect is
returning to initial state and shuts down. For example, slightly
dragging and releasing a thumbnail such that both TapHandler & DragHandler
would react and play each their own transition (yes, it's another bug);
or by pressing Escape key while dragging.