To reproduce this bug:
1. Import the first contig (pkF70) of Klebsislla.sort.bam.
2. Open the file with reference sequence (pkF3_all.fa).
3. Associate the first contig of sequence with assembly.
4. Remove the first contig from sequence.
The first problem is: there is not the question about unassociate.
5. Remove whole sequence.
6. If you click 'No' in the dialog of saving, ugene crashes.