Impossible to add sequence with different alphabet to alignment using the project dialog

Scenario:

1. Open "_common_data\fasta\amino_ext.fa".
2. Open "samples\clustalw\COI.aln".
3. In the context menu of the sequences area select "Add->Sequence from current project".
   Expected state: it is possible to select a sequence from the "amino_ext.fa" file.
   Actual state: there is nothing to select.