Visualizing ConvNet filters using my own fine-tuned network resulting in a "NoneType" when running: K.gradients(loss, model.input)[0]
SOLVED:  I had to use: grads = K.gradients(loss, model.get_layer(my_Model_object).get_layer('input_1').input)[0] 

instead of just grads = K.gradients(loss, model.input)[0] 

which is confusing because both model.get_layer(my_Model_object).get_layer('input_1').input)[0] 
and 
model.input[0] print the same thing and are of the same type.