convert to onnx fails

[liamsun2019]

Hi author,

When I convert the model to onnx format, an error similar with following rises up:
ValueError: Graph disconnected: cannot obtain value for tensor Tensor("joints/conv2d_58_input:0", shape=(None, 2, 2, 288), dtype=float32) at layer "conv2d_58". The following previous layers were accessed without issue: []

Both tensorflow 2.3.0 and 2.5.0 have the same error. I did not find out ant possible reasons for this issue. Any suggestions? Thanks.