Callback that is invoked when the opening of a model is complete. If there were no errors, the ModelDirectory will have been populated once this is called.

If you need to do any async calls or want to do something else before the streaming starts, return POSTPONE_STREAMING from this callback. When you are done and would like the model to start the initial streaming, call the startPostponedStreaming() function.

If you return nothing or START_STREAMING the streaming of the model will start immediately.

Note: DO NOT do any async calls in the callback unless you return POSTPONE_STREAMING!