Vaadin Widgetset编译错误后迁移到Vaadin8

I just migrated my Vaadin project from 7.7.6 to 8.0.5. First I wanted to use the Vaadin8 migration tool but my IDE used too many vaadin.* imports so I just did it manually. After hours of work (compatibility package imports) the maven build worked again and I wanted to open the WebApp in my browser but I got the "Failed to load the Widgetset..." error message. I thought that I have to recompile the Widgetset but the vaadin-maven-plugin throws an error:I just migrated my Vaadin project from 7.7.6 to