What have you been doing?
I have been trying to do my homework.
That's fine, but note that we do have an implied time reference - the present moment, and the period leading up to it.
I cannot imagine using a so-called 'future perfect continuous' without some form of time reference. 'I will have been working' on its own means nothing, unless we know the future time up to which the working will have been in progress.
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