Hi,
i want to scrape this table [1]. But it doesnt work correctly, cause it often scrapes "null". Here is my sitemap. Whats wrong? Thank you for help 
{"_id":"gebietsaenderungen_sachsen","startUrl":["https://de.wikipedia.org/wiki/Liste_der_Gebietsänderungen_in_Sachsen"],"selectors":[{"id":"Jahr","type":"SelectorLink","parentSelectors":["_root"],"selector":"tr:nth-of-type(n+3) th a","multiple":true,"delay":0},{"id":"gemeinden","type":"SelectorTable","parentSelectors":["Jahr"],"selector":"table.wikitable","multiple":true,"columns":[{"header":"Datum","name":"Datum","extract":true},{"header":"Gemeinde vor der Änderung","name":"Gemeinde vor der Änderung","extract":true},{"header":"Gebietsänderung","name":"Gebietsänderung","extract":true},{"header":"Gemeinde nach der Änderung","name":"Gemeinde nach der Änderung","extract":true},{"header":"Landkreis","name":"Landkreis","extract":true}],"delay":0,"tableDataRowSelector":"tr:nth-of-type(n+2)","tableHeaderRowSelector":"tr:nth-of-type(1)"}]}
[1] https://de.wikipedia.org/wiki/Liste_der_Gebietsänderungen_in_Sachsen_1991