UGENE crashes on closing sequence view with active "Find enzymes" task

Scenario to reproduce:

1. Open "test/_common_data/fasta/human_T1_cutted.fa".
2. Click the "Find restriction sites" button on the main toolbar.
3. Select a single enzyme: "AbaBGI". Start the search.
4. Close the sequence view until task has finished.
   Expected state: the task is canceled.
   Current state: UGENE crashes.