The reason why you see that null one(s) is because you select the whole column(s) as data source (ex. A:C) instead of only selecting the range with data (ex. A1: C20).
You have 2 options:
1. by selecting A:C, you have to add a filter in chart setup section to exclude those rows without data
2. change the data source range to A1:C20
The 1st is better as it'll be able to handle the potential change of the data.