title: ValueError: Attempt to convert a value (None) with an unsupported type (<class 'NoneType'>) to a Tensor
body: <p>This error appears when i try load model with my custom layer.</p>
  <pre><code>model = load_model('my_model.h5', custom_objects={'MyLayer': MyLayer})
  </code></pre>
  <p>Im trying to visualize what is happening in my custom keras layer. Im working with images of BrainMRI and trying just to flip them in layer.</p>
  <pre><code>import tensorflow as tf
  from keras import backend as K
  from keras.layers import Layer
  import matplotlib.pyplot as plt
  
  
  class MyLayer(Layer):
      def __init__(self, num_outputs, name = 'MyLayer', **kwargs):
          super(MyLayer, self).__init__(**kwargs)
          self.num_outputs = num_outputs
  
  
      def build(self, input_shape):
          self.kernel = self.add_weight(name='kernel',
                                    shape=(input_shape[1], self.num_outputs),
                                    initializer='normal', trainable=True)
  
          super(MyLayer, self).build(input_shape)
  
      def call(self, inputs, **kwargs):
          flipped = K.reverse(inputs,axes=0)
          return flipped
  
      def compute_output_shape(self, input_shape):
          return (input_shape[0], self.output_dim)
  
      def get_config(self):
          base = super(MyLayer, self).get_config()
          base['num_outputs'] = self.num_outputs
          return dict(list(base.items()))
  </code></pre>
  <p>I tried reinstall keras as some thread was recommending but nothing happens</p>