Axis 3 out of bounds #1

when running the following code in TrainModel.ipynb an error arose:
[sample_batch_train_data, sample_batch_test_data] = next(train_generator)

the error is:

ValueError Traceback (most recent call last)
in ()
24 plt.show()
25
---> 26 [sample_batch_train_data, sample_batch_test_data] = next(train_generator)
27 for i in range(0, 3, 1):
28 draw_image_with_label(sample_batch_train_data[0][i], sample_batch_test_data[i])
D:\tools\anaconda\envs\ND-P2-python3\lib\site-packages\keras\preprocessing\image.py in next(self, *args, **kwargs)
735
736 def next(self, *args, **kwargs):
--> 737 return self.next(*args, **kwargs)
738
739
~\AutonomousDrivingCookbook\AirSimE2EDeepLearning\Generator.py in next(self)
240 # so it can be done in parallel
241
--> 242 return self.__get_indexes(index_array)
243
244 def __get_indexes(self, index_array):
~\AutonomousDrivingCookbook\AirSimE2EDeepLearning\Generator.py in __get_indexes(self, index_array)
243
244 def __get_indexes(self, index_array):
--> 245 index_array = sorted(index_array)
246 if self.x_prev_states is not None:
247 batch_x_images = np.zeros(tuple([self.batch_size]+ list(self.x_images.shape)[1:]),
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

please note that I have not added or deleted any code in Generator.py file. And I have generated datasets successfully in step 0.
I do not know why this error arose, it makes no sense that sorted() function made this error.