An unloaded sequence can't be used for the dotplot building

Scenario to reproduce:

1. Open "data/samples/Genbank/murine.gb".
2. Unload the document "murine.gb".
3. Open "data/samples/FASTA/human_T1.fa".
4. Click the "Build dotplot" button on the toolbar.
   Expected state: a "DotPlot" dialog appears, only one sequence is available in the sequence selectors.
5. Click the "Load sequence" button in the dialog.
6. Select "data/samples/Genbank/murine.gb".
   Expected state: the "murine.gb" document is loaded, the sequence is used to build a dotplot.
   Current state: a message that the document is already present in the project appears, the document is not loaded.