SAM file can't be imported if reference file can't be copied to the destination folder

Scenario to reproduce:

1. Create a folder somewhere.
2. Create a file "chrM.fa" in the folder. The file can be empty.
3. Open "data/samples/Assembly/chrM.sam".
   Expected state: "Import SAM file" dialog appears.
4. Set "data/samples/Assembly/chrM.fa" as reference, save the result to the created folder. Accept the dialog.
   Expected state: the assembly is successfully imported.
   Current state: the import task fails, because it can't copy a file with reference sequence to the destination folder.