XGBoostError when loading model from json file

I'm trying to load a trained XGBoost model which has been saved in a json file. I'm using the following code:

params= {'objective' : 'multi:softmax',
         'eval_metric': 'mlogloss',
         'num_class': 10,
         'early_stopping_rounds': 10}

xgb = xgb.XGBClassifier(**params)
xgb.load_model("xgb_default.json")

However I'm getting an error. I will include it in here together with the Traceback:

XGBoostError                              Traceback (most recent call last)
<ipython-input-4-8a9abeb40a78> in <module>
     10 
     11 xgb = xgb.XGBClassifier(**params)
---> 12 xgb.load_model("xgb_default.json")

~\anaconda3\lib\site-packages\xgboost\sklearn.py in load_model(self, fname)
    412         if not hasattr(self, '_Booster'):
    413             self._Booster = Booster({'n_jobs': self.n_jobs})
--> 414         self._Booster.load_model(fname)
    415         meta = self._Booster.attr('scikit_learn')
    416         if meta is None:

~\anaconda3\lib\site-packages\xgboost\core.py in load_model(self, fname)
   1601             # assume file name, cannot use os.path.exist to check, file can be
   1602             # from URL.
-> 1603             _check_call(_LIB.XGBoosterLoadModel(
   1604                 self.handle, c_str(os_fspath(fname))))
   1605         elif isinstance(fname, bytearray):

~\anaconda3\lib\site-packages\xgboost\core.py in _check_call(ret)
    186     """
    187     if ret != 0:
--> 188         raise XGBoostError(py_str(_LIB.XGBGetLastError()))
    189 
    190 

XGBoostError: [11:07:00] C:\Users\Administrator\workspace\xgboost-win64_release_1.2.0\include\xgboost/json.h:65: Invalid cast, from Null to Array

Does anyone know what is the issue here? Thank you in advance!