best_model_for_estimator() returns empty objects for some estimators

Hi @sonichi:

I just found out that after running automl with several customized estimators, for certain estimators, best_model_for_estimator() returned an empty object although a few models have been built with them. When I tried to save the best model, there were error messages like below:

AttributeError: 'NoneType' object has no attribute 'save_model'

Do you know why? My team has been using FLAML for quite a while so it is not due to coding errors or time budget. The dataset is big; however, we used similar datasets with no issue before. Our FLAML is the latest version, 0.9.5 and we set model_history = True.

Thank you.