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

[Utkarsh]

How do I fix the following error? It happens when I try to run the final codeblock where you have to call the function. Please help, I'm kinda stuck here

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-13-1733272e84a3> in <module>
----> 1 hist = train_mnist(x_train, y_train)

<ipython-input-12-1769252edc00> in train_mnist(x_train, y_train)
     23     # Fit the model for 10 epochs adding the callbacks
     24     # and save the training history
---> 25     history = model.fit(x_train, y_train, epochs=10, callbacks=[callbacks])
     26 
     27     ### END CODE HERE

/opt/conda/lib/python3.8/site-packages/keras/utils/traceback_utils.py in error_handler(*args, **kwargs)
     65     except Exception as e:  # pylint: disable=broad-except
     66       filtered_tb = _process_traceback_frames(e.__traceback__)
---> 67       raise e.with_traceback(filtered_tb) from None
     68     finally:
     69       del filtered_tb

/opt/conda/lib/python3.8/site-packages/keras/callbacks.py in set_model(self, model)
    283       model.history = self._history
    284     for callback in self.callbacks:
--> 285       callback.set_model(model)
    286 
    287   def _call_batch_hook(self, mode, hook, batch, logs=None):

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