NullPointerException with webjars resources in the log when exporting a page in HTML

2018-02-22 08:49:42,374 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [bootstrap-switch], path = [css/bootstrap3/bootstrap-switch.min.css]. Root cause = [NullPointerException: ] 
2018-02-22 08:49:42,379 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [bootstrap-switch], path = [css/bootstrap3/bootstrap-switch.min.css]. Root cause = [NullPointerException: ] 
2018-02-22 08:49:42,384 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [bootstrap-switch], path = [css/bootstrap3/bootstrap-switch.min.css]. Root cause = [NullPointerException: ] 
2018-02-22 08:49:42,388 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [bootstrap-switch], path = [css/bootstrap3/bootstrap-switch.min.css]. Root cause = [NullPointerException: ] 
2018-02-22 08:49:42,451 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [org.xwiki.platform:xwiki-platform-tree-webjar], path = [tree.min.css]. Root cause = [NullPointerException: ] 
2018-02-22 08:49:42,456 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [org.xwiki.platform:xwiki-platform-tree-webjar], path = [tree.min.css]. Root cause = [NullPointerException: ] 
2018-02-22 08:49:42,563 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [org.xwiki.platform:xwiki-platform-tree-webjar], path = [tree.min.css]. Root cause = [NullPointerException: ] 
2018-02-22 08:49:42,642 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [org.xwiki.platform:xwiki-platform-tree-webjar], path = [tree.min.css]. Root cause = [NullPointerException: ] 
2018-02-22 08:49:42,677 [http://127.0.0.1:8080/xwiki/bin/export/Main/WebHome?format=html&pages=xwiki%3AMain.WebHome] WARN  o.x.w.s.WebJarsScriptService   - Error while serializing WebJar URL for id [bootstrap-tour], path = [css/bootstrap-tour.min.css]. Root cause = [NullPointerException: ]