Based on the error message it seems that netD is not returning a probability for each sample in the batch (which would have the shape [batch_size=128]), but instead an output of [batch_size=128, 1, 12, 12] due to the increased spatial size. After the view(-1) operation this would create a tensor of 128*12*12=18432.
You would have to make sure the output is a single pixel, e.g. by adding more layers (pooling, strided convs etc.).