tree/view: do not attempt enter on hidden child

If a subsurface is created for a surface that is associated with a
scratchpad hidden view, do not attempt to send an enter to it. The
subsurface is not on any output and since there is no workspace
associated with the view, attempting to get the output for the NULL
workspace will result in a SIGSEGV.

sway/tree/view.c

@@ -896,8 +896,10 @@ void view_child_init(struct sway_view_child *child,
 	wl_signal_add(&view->events.unmap, &child->view_unmap);
 	child->view_unmap.notify = view_child_handle_view_unmap;
 
-	struct sway_output *output = child->view->container->workspace->output;
-	wlr_surface_send_enter(child->surface, output->wlr_output);
+	struct sway_workspace *workspace = child->view->container->workspace;
+	if (workspace) {
+		wlr_surface_send_enter(child->surface, workspace->output->wlr_output);
+	}
 
 	view_init_subsurfaces(child->view, surface);
 }