Repair geometry errors
From AWF-Wiki
- Click the Open Data Source Manager
and then Add vector layer 
and select the file and select the file geodata_w01\VG250_GEM_KRS_Goettingen.shp.
- In the search engine of the Processing Toolbox, type clean and select v.clean under vector of GRASS.
- Select the vector file as the input layer and click Run.
