Slice Assembly error

I got an issues in slice assembly tool.
Its show the error - java.lang.nullpointer expression.
please give a solution for this.

Can you provide more details of the problem? For example, the product name and processing steps etc.

Sentinel-1 A SLC products downloded from ASF
The processing chain went as following.

Each written file is an interferogram formed of three bursts, one has bursts No. 1,2,3 and the other has bursts No, 4,5,6 from the same sub-swath. I put both files in S-1 TOPS Slice Assembly Operator and this error popped up