I have trouble splitting a multipart feature and can't figure out what I'm doing wrong. I have a multipart feature of Islands which all belong to the same country, so they should stay a multipart feature. However, in some cases the shapefile wasn't digitized properly so there is one larger polygon that in reality should be a few little ones. I tried using the "Split Parts" tool and it works for the first split so that I have two instead of one polygon, but when I try to split it again I get an error saying the geometry is invalid and I need to fix it before I can split.