text: Title: Fitting autokeras with the EarlyStopping baseline parameter does not work
  Body: ### Bug Description
  When the EarlyStopping baseline parameter is triggered, autokeras is crashing with the following error : TypeError: object of type 'NoneType' has no len()
  
  ### Bug Reproduction
  Here is the colab:
  https://colab.research.google.com/drive/1oqxaIaXb51qGaFSRBJtUL5yDZ77Udjx1?usp=sharing
  
  
  ### Setup Details
  Include the details about the versions of:
   - OS type and version:
   - Python: 
   - autokeras: 1.0.12
   - keras-tuner: master
   - scikit-learn:
   - numpy:
   - pandas:
   - tensorflow:
  