"Build dotplot" crashes when an empty sequence is provided

0. Create a file with an empty sequence. A FASTA file with the first line ">seq1" and the empty second line
1. Open any sequence
2. On the toolbar, press "Build dotplot"
3. Press "Load sequence" in the dialog. Load the empty sequence
4. Select the empty sequence as the second sequence in the combobox

Bug: UGENE crashes
Expected: an error message is shown

The solution of the bug must be complex. It crashes because "getAlphabet()" returns NULL. So find all usages of this methods and provide adequate actions if this method returns NULL