Load remote document and open view task can't be cancelled

Scenario to reproduce:

1. Select {File -> Access remote database...}

   menu item in the main menu.

2. Fill the dialog:
   Resource ID: NW_003943623;
   Database: NCBI GenBank (DNA sequence);
   Force download the appropriate sequence: checked
   and accept the dialog.
3. Try to cancel the task.
   Expected state: the task cancels within a half of a minute.
   Current state: the task doesn't cancel.