in my flutter project I cannot open a file in design editor, it says: "Design editor is unavailable until next Gradle sync."

So I looked the error up, and it seems, that I have to manually sync the files. But I cannot find the option. It should be under File - Sync Project with Gradle Files (Shortcut shift+command+o) But I don't have that Option (see screenshot). Also the shortcut does not work.

But I can see it in the settings:

Am I missing something?

Edit: here is a list of my root directory: