Crash on removing document which is used for opening view

Scenario to reproduce:

1. Open "test/_common_data/clustal/10000_sequences.aln".
2. Call context menu on the "10000_sequences" object.
3. Select {Export/Import -> Export alignmnet to sequence format...}

   menu item.

4. Export somewhere as fasta, sure that "Add document to the project" checkbox is checked.
5. Wait until open view task will start.
   Expected state: the fasta document is present in the project, open view task is in progress.
6. Delete the fasta document from the project.
   Current state: UGENE crashes.

The issue is based on the crash report 5691.