There appear to be two 3rd party solutions.
There is this one
which appears to be free.
There is also this one
which is not free. It states it was written by RBoys with official help and approval of Nuki.
So, it seems Nuki rather than providing an official integration themselves which we might have expected to be free have instead handed responsibility to a professional Smartthings developer who makes his living selling Smartthings handlers. Now one cannot really complain RBoys is charging for his work, this is how he makes a living but it does suggest laziness by Nuki in not doing their own.