Edited later to add:
The original press release seems to be gone now, but here is what is claimed to be the revised release, and an article on the same topic.
https://www1.wdr.de/nachrichten/polizei-dortmund-greta-gewaltbereit-100.html
https://www.yahoo.com/news/german-politician-calls-greta-thunberg-102611513.html
A lot of Germans still remember the 1972 Summer Olympics massacre in Munich and the aftermath like the plane hijack to free the captured perpetrators and are therefore biased against Palestinians, suspecting a lot of violent people among them. It’s not a good environment to protest.
German here: if you take peaceful demonstration as an option away from people, they are left with two options: being resigned or non-peaceful demonstration. The German stance to hinder peaceful demonstration against the Israeli atrocities is stupid and labeling Greta Thunberg as primed for violence is beyond stupid. And the majority of Germans has no active memory of the 72 Olympics incident.