The added layer must be an instance of class Layer. Found:
How did you import "BatchNormalization" layer?
I had the same error and the solution was changing the way I was importing: 

from tensorflow.keras.layers import Activation, Dropout, Dense, Conv2D, Flatten, Dropout, MaxPooling2D, BatchNormalization


I put the BatchNormalization with all the other layers, and it work in my case. 

Hope it is useful.