It's not about the Ukrainians directly. On the more 'civil' side of this discourse it's mostly claims that this will become a general asylum or registration center (which it won't) and/or that there'll be a lot of crime, often accompanied by sources featuring African people.
This flows directly into the less civil side of the discussion, which ranges from conspiracy theories of EU or WEF-planned repopulation to all out etno-nationalism. Our city has a rich history of voting literal nazis into the council. There's talk of storming the city council and all that.