Crash on removing big alignment from project after export

The problem appeared after UGENE-5705 fix.
There is a crash on removing an alignment with tree from the project.

Affected test:
GUITest_regression_scenarios_test_4009
The test failed on Ubuntu and Widows starting 14/04/2018. The test passed on macOS.

Scenario:

1. Open file "_common_data\clustal\big.aln"
2. Open the "Export Consensus" OP tab.
3. Click the "Export" button.
4. Remove the "big.aln" document.
   Expected state: the document has been removed.
   Actual state: there is a crash (in the ExtractConsensusTask procedure).