Crash on building the second tree when the first tree is unloaded

The issue was detected by GUI regression test 1568 in build 6.4.2017.
The issue is not reproduced on release 1.26.1.

Steps for reproducing:

1. Open "samples/CLUSTALW/COI.aln".
2. Build a tree using the option panel.
3. Select the created tree document and unload it.
4. Build a tree again as on step 2.
   Current result:
   Crash on adding the new document to the project:

   Adding document to the project: D:/ugene/ugene/data/samples/CLUSTALW/COI24.nwk
   SQL: error: Unexpected query result code: 19 (foreign key constraint failed) in query: INSERT INTO ObjectRelation (object, reference, role) VALUES(?1, ?2, ?3)
   Unexpected query result code: 19 (foreign key constraint failed)
   SQL: error: SQLite: Error finalizing statement: Error querying database: foreign key constraint failed in query: INSERT INTO ObjectRelation (object, reference, role) VALUES(?1, ?2, ?3)
   Trying to recover from error: Unexpected query result code: 19 (foreign key cons traint failed) at src\models\GObject.cpp:198
   Assertion failed: 0, file src\models\GObject.cpp,line198