libsysprof: be defensive against null traceable

src/libsysprof/sysprof-callgraph.c

@@ -461,6 +461,13 @@ sysprof_callgraph_new_worker (GTask        *task,
     {
       g_autoptr(SysprofDocumentTraceable) traceable = g_list_model_get_item (self->traceables, i);
 
+      /* Currently, we might be racing against changes in the model. And we
+       * definitely need to address that. But this at least keeps things
+       * recovering in the meantime.
+       */
+      if (traceable == NULL)
+        break;
+
       sysprof_callgraph_add_traceable (self, traceable, i);
     }