chapter-9, Name Scopes valueerror. cannot be interpreted as a Tensor

```
------------------------------------------------------------------------
ValueError Traceback (most recent call last)
~/anaconda3/lib/python3.7/site-packages/tensorflow/python/client/session.py in __init__(self, fetches, contraction_fn)
302 self._unique_fetches.append(ops.get_default_graph().as_graph_element(
--> 303 fetch, allow_tensor=True, allow_operation=True))
304 except TypeError as e:

~/anaconda3/lib/python3.7/site-packages/tensorflow/python/framework/ops.py in as_graph_element(self, obj, allow_tensor, allow_operation)
3795 with self._lock:
-> 3796 return self._as_graph_element_locked(obj, allow_tensor, allow_operation)
3797

~/anaconda3/lib/python3.7/site-packages/tensorflow/python/framework/ops.py in _as_graph_element_locked(self, obj, allow_tensor, allow_operation)
3879 if obj.graph is not self:
-> 3880 raise ValueError("Operation %s is not an element of this graph." % obj)
3881 return obj

ValueError: Operation name: "GradientDescent"
op: "NoOp"
input: "^GradientDescent/update_theta/ApplyGradientDescent"
is not an element of this graph.

During handling of the above exception, another exception occurred:

ValueError Traceback (most recent call last)
in
13 step = epoch * n_batches + batch_index
14 file_writer.add_summary(summary_str, step)
---> 15 sess.run(training_op, feed_dict={X: X_batch, y: y_batch})
16
17 best_theta = theta.eval()

~/anaconda3/lib/python3.7/site-packages/tensorflow/python/client/session.py in run(self, fetches, feed_dict, options, run_metadata)
948 try:
949 result = self._run(None, fetches, feed_dict, options_ptr,
--> 950 run_metadata_ptr)
951 if run_metadata:
952 proto_data = tf_session.TF_GetBuffer(run_metadata_ptr)

~/anaconda3/lib/python3.7/site-packages/tensorflow/python/client/session.py in _run(self, handle, fetches, feed_dict, options, run_metadata)
1156 # Create a fetch handler to take care of the structure of fetches.
1157 fetch_handler = _FetchHandler(
-> 1158 self._graph, fetches, feed_dict_tensor, feed_handles=feed_handles)
1159
1160 # Run request and get response.

~/anaconda3/lib/python3.7/site-packages/tensorflow/python/client/session.py in __init__(self, graph, fetches, feeds, feed_handles)
472 """
473 with graph.as_default():
--> 474 self._fetch_mapper = _FetchMapper.for_fetch(fetches)
475 self._fetches = []
476 self._targets = []

~/anaconda3/lib/python3.7/site-packages/tensorflow/python/client/session.py in for_fetch(fetch)
272 if isinstance(fetch, tensor_type):
273 fetches, contraction_fn = fetch_fn(fetch)
--> 274 return _ElementFetchMapper(fetches, contraction_fn)
275 # Did not find anything.
276 raise TypeError('Fetch argument %r has invalid type %r' % (fetch,

~/anaconda3/lib/python3.7/site-packages/tensorflow/python/client/session.py in __init__(self, fetches, contraction_fn)
308 except ValueError as e:
309 raise ValueError('Fetch argument %r cannot be interpreted as a '
--> 310 'Tensor. (%s)' % (fetch, str(e)))
311 except KeyError as e:
312 raise ValueError('Fetch argument %r cannot be interpreted as a '

ValueError: Fetch argument cannot be interpreted as a Tensor. (Operation name: "GradientDescent"
op: "NoOp"
input: "^GradientDescent/update_theta/ApplyGradientDescent"
is not an element of this graph.)

```
i was reset the previous graph. but it is throwing again for cannot be interpreted as a Tensor

How to resolve this issue sir.

Thank you