sculpt manager: prevent double destruction

The '_currently_constructed' pointer caches the information about which
'Launched_child' is currently configured in the menu. When discarding
the runtime (e.g., when un-using a file system) at this point, this
cached pointer was not invalidated while all 'Launched_child' objects
would be freed (including the currently constructed one). On the next
attempt to construct a new child, the sculpt manager attempted to
destruct the 'Launched_child' referred by the (now outdated)
'_currently_constructed' again.

Fixes #3240

repos/gems/src/app/sculpt_manager/model/runtime_state.h

@@ -396,6 +396,12 @@ class Sculpt::Runtime_state : public Runtime_info
 
 		void reset_abandoned_and_launched_children()
 		{
+			/*
+			 * Invalidate '_currently_constructed' pointer, which may point
+			 * to a to-be-destructed 'Launched_child'.
+			 */
+			discard_construction();
+
 			_abandoned_children.for_each([&] (Abandoned_child &child) {
 				destroy(_alloc, &child); });