I think it would have to be decided by the ECB but I would argue that like the big 5, if you put in extra money then you should get automatic place in the final. It would be hard to argue against that given the reasons the big 5 get their spots.
The Netherlands hosted for Israel in 1980 but thats the days before the semi finals started:
Via Wiki:
Israel, the winner of the 1979 contest, declined to host it for the second time in a row, as the IBA could not fund another international production, and the Israeli government turned down a request to extend the IBA budget. The European Broadcasting Union also set the broadcast on the same day as the Yom HaZikaron holiday, which meant Israel decided not to participate at all, marking the first and only time that the previous year's winner did not even compete. After Spain, the 2nd-place winner of 1979, and reportedly the UK, both declined to host, the Netherlands finally agreed to host the show in a small-scale production. According to Yair Lapid, son of Tommy Lapid who was then the IBA director general, Lapid called his counterpart at NOS and convinced him to take the "undesired honour", when he realised that the extra cost could paralyse the regular work of the IBA. This remains the last time the previous year's winning country did not host the contest the year after.
So Spain as 2nd place were offered it first, then the UK (this was before the Big 5) as one of the countries most able to host it.
So the decision looks like its primarily down to the EBU to offer, but it would be polite for them to work with Ukraine, particularly under the circumstances.
It is going to be an additional thing for Zelensky to sort out, but it does sell Ukraine to the world and I think regardless of who hosts there will be a strong cooperation with Ukraine over the show's content.
I think its quite exciting for whoever gets to do it tbh. Its just the £££.