Strange behaviour of Similarity label.

Scenario to reproduce:
1. Open "data/samples/CLUSTALW/ty3.aln.gz"
2. Add an addition sequence from the file "data/samples/Genbank/PBR322.gb"
3. Open "Pairwise Alignment" options panel tab
4. Set first sequence from alignment ty3.aln.gz (CopCinTy3-9)
5. Set second sequence from PBR322.gb (SYNPBR322)
Expected state: Similatiry calculated and value is 4%
Curent state: Same as expected.
6. Close options panel
7. Undo changes
8. Open "Pairwise Alignment" options panel tab
Expected state: Second field os sequences is empty
Curent state: Same as expected.
9. Redo changes or again add an addition sequence from the file "data/samples/Genbank/PBR322.gb"
Expected state: Second field is sequences is empty. Similarity label not showed.
Curent state: Second field is sequences is empty. But similarity label is showed. Similarity value is 4%.
10. Set other seaquence as the first sequence of pairvise alignment
Expected state: Second field os sequences is empty. Similarity label not showed.
Curent state: Second field os sequences is empty. Similarity label is showed. Similarity value was recalculated.

10. Open "Highlighting" options panel tab
Expected state: "Color" and "Highlighting" not changed
Curent state: "Color" value is "No color". And "Highlighting" value is "No highlighting"