Sort Null value in column by another column

Hello there

I am using matrix visual, so I have 3 fileds in Row section.

Category A

Category A1

Category A1.1

Category A1.2

Category A2

Category A2.1

Category A2.2

Category B

Category B1

Category B1.1

Category B1.2

Category B2

Category B2.1

Category B2.2 and so on like 10 Categories total.

Now Category A, B, C etc can't be empty, But Category A1, A2, B1, B2 can be empty.. Category A1.1 A1.2 B1.1 B1.2 etc can never be empty. 

Now the problem is when A1 A2 B1 B2 etc are empty but their sub catering are not empty. I can't sort them in order.

when I try to sort my 2nd category column by 3rd category coulumn, it doesn't allow me to sort any data because 2nd Category have many null values but 3rd category have no nulls. So when I sort it, it gives the error "2nd category can't be sorted by 3rd, because 3rd Category can't have multiple values for same value of 2nd Category"... 

So question is, how can I sort null values of one column bases on values of other column... Against every null value, there is a difference value in other column... Please help