title: AttributeError: 'NoneType' object has no attribute '_inbound_nodes' while trying to add multiple keras Dense layers
body: <p>The input are 3 independent channels of 1000 features. I'm trying to pass each channel through a independent NN path, then concatenate them into a flat layer. Then apply a FCN on the flatten layer for a binary classification. 
  I'm trying to add multiple Dense layers together, like this:</p>
  
  <p>def tst_1():</p>
  
  <pre><code>inputs = Input((3, 1000, 1))
  
  dense10 = Dense(224, activation='relu')(inputs[0,:,1])
  dense11 = Dense(112, activation='relu')(dense10)
  dense12 = Dense(56, activation='relu')(dense11)
  
  dense20 = Dense(224, activation='relu')(inputs[1,:,1])
  dense21 = Dense(112, activation='relu')(dense20)
  dense22 = Dense(56, activation='relu')(dense21)
  
  dense30 = Dense(224, activation='relu')(inputs[2,:,1])
  dense31 = Dense(112, activation='relu')(dense30)
  dense32 = Dense(56, activation='relu')(dense31)
  
  flat = keras.layers.Add()([dense12, dense22, dense32])
  
  dense1 = Dense(224, activation='relu')(flat)
  drop1 = Dropout(0.5)(dense1)
  dense2 = Dense(112, activation='relu')(drop1)
  drop2 = Dropout(0.5)(dense2)
  dense3 = Dense(32, activation='relu')(drop2)
  densef = Dense(1, activation='sigmoid')(dense3)
  
  model = Model(inputs = inputs, outputs = densef)
  
  model.compile(optimizer=Adam(), loss='binary_crossentropy', metrics=['accuracy'])
  
  return model
  model = tst_1()
  
  model.summary()
  </code></pre>
  
  <p>but I got this error:</p>
  
  <blockquote>
    <p>/usr/local/lib/python2.7/dist-packages/keras/engine/network.pyc in build_map(tensor, finished_nodes, nodes_in_progress, layer, node_index, tensor_index)
       1310             ValueError: if a cycle is detected.
       1311         """
    -> 1312         node = layer._inbound_nodes[node_index]
       1313 
       1314         # Prevent cycles.</p>
    
    <p>AttributeError: 'NoneType' object has no attribute '_inbound_nodes'</p>
  </blockquote>