I suspect that this is happening because ui_task is being captured into the task posted to the concurrent task runner. ui_task is a shared object created with fml::MakeCopyable, and it will be destructed on whatever thread drops the last reference.

ui_task captures some DartPersistentValue objects that will be deleted when ui_task is destructed. The DartPersistentValue destructor will try to enter the associated Dart isolate and invoke Dart APIs. This will fail if the destructor is running on a background thread but the UI thread has already entered that isolate.

Try capturing the DartPersistentValue objects as raw pointers and then rewrapping them in unique_ptr objects within the code that is explicitly scheduled on the UI thread.