Bert Embeddings Layer raises `TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'` with BiLSTM

I've problems integrating Bert Embeddings Layer in a BiLSTM model for word sense disambiguation task,

windows 10
python 3.6.4
tenorflow 1.12
keras 2.2.4
No virtual environments were used

def baseline_model(output_size, max_seq_len=64, visualize=False, plot=False):
    hidden_size = 128
    # Output size: is the number of words and senses as per my dataset, which is 58_809
    in_id = Input(shape=(max_seq_len,), name="input_ids")
    in_mask = Input(shape=(max_seq_len,), name="input_masks")
    in_segment = Input(shape=(max_seq_len,), name="segment_ids")
    bert_inputs = [in_id, in_mask, in_segment]

    bert_output = BertEmbeddingLayer(n_fine_tune_layers=3, pooling="first")(bert_inputs)

    bilstm = Bidirectional(LSTM(hidden_size, dropout=0.2,
                                recurrent_dropout=0.2,
                                return_sequences=True)
                           )(bert_output)

    output = TimeDistributed(Dense(output_size, activation='softmax'))(bilstm)

    mdl = Model(inputs=bert_inputs, outputs=output, name="Bert_BiLSTM")

    mdl.compile(loss="sparse_categorical_crossentropy",
                optimizer="adam", metrics=["acc"])

    visualize_plot_mdl(visualize, plot, mdl)

    return mdl

The layer BertEmbeddingLayer() is imported from strongio/keras-bert, as well as following the approach in the file to integrate my work however I always have this error, please check the traceback below (exception is raised when building the model)

Traceback (most recent call last):
  File "code/prova_bert.py", line 230, in <module>
    model = baseline_model(output_size, max_seq_len, visualize=True)
  File "code/prova_bert.py", line 165, in baseline_model
    )(bert_embeddings)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\layers\wrappers.py", line 473, in __call__
    return super(Bidirectional, self).__call__(inputs, **kwargs)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\engine\base_layer.py", line 746, in __call__
    self.build(input_shapes)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\layers\wrappers.py", line 612, in build
    self.forward_layer.build(input_shape)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\utils\tf_utils.py", line 149, in wrapper
    output_shape = fn(instance, input_shape)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\layers\recurrent.py", line 552, in build
    self.cell.build(step_input_shape)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\utils\tf_utils.py", line 149, in wrapper
    output_shape = fn(instance, input_shape)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\layers\recurrent.py", line 1934, in build
    constraint=self.kernel_constraint)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\engine\base_layer.py", line 609, in add_weight
    aggregation=aggregation)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\training\checkpointable\base.py", line 639, in _add_variable_with_custom_getter
    **kwargs_for_getter)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\engine\base_layer.py", line 1977, in make_variable
    aggregation=aggregation)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\ops\variables.py", line 183, in __call__
    return cls._variable_v1_call(*args, **kwargs)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\ops\variables.py", line 146, in _variable_v1_call
    aggregation=aggregation)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\ops\variables.py", line 125, in <lambda>
    previous_getter = lambda **kwargs: default_variable_creator(None, **kwargs)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\ops\variable_scope.py", line 2437, in default_variable_creator
    import_scope=import_scope)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\ops\variables.py", line 187, in __call__
    return super(VariableMetaclass, cls).__call__(*args, **kwargs)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\ops\resource_variable_ops.py", line 297, in __init__
    constraint=constraint)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\ops\resource_variable_ops.py", line 409, in _init_from_args
    initial_value() if init_from_fn else initial_value,
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\keras\engine\base_layer.py", line 1959, in <lambda>
    shape, dtype=dtype, partition_info=partition_info)
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\ops\init_ops.py", line 473, in __call__
    scale /= max(1., (fan_in + fan_out) / 2.)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
Exception ignored in: <bound method BaseSession.__del__ of <tensorflow.python.client.session.Session object at 0x0000026396AD0630>>
Traceback (most recent call last):
  File "C:\Users\Sheikh\AppData\Local\Programs\Python\Python36\Lib\site-packages\tensorflow\python\client\session.py", line 738, in __del__
TypeError: 'NoneType' object is not callable

Please refer to my issue on their repo and for data examples being fed to the model please check this issue

from Bert Embeddings Layer raises `TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'` with BiLSTM