Error loop on filtering in a shared database

The issue is reproduced on the master build 23.08.17.

Scenario:

1. Select "File->Connect to UGENE shared database".
2. Select "UGENE public database" in the dialog.
3. Click "Connect".
4. Type "human" in the search area in the Project View.
   Expected state: correct search result without any errors are shown.
   Actual state: There is endless loop of errors.

   [ERROR][15:53] Cannot start a transaction
   [ERROR][15:53] Trying to recover from error: Internal error! The object unexpectedly does not exist: Sequence alphabet at src\misc\SequenceContentFilterTask.cpp:135
   [ERROR][15:53] Trying to recover from error: Unable to prepare search algorithm at src\misc\SequenceContentFilterTask.cpp:49
   

The following two tasks are in the "Running" state:

• Task {Filtering project content by the "Sequence content" criterion}
• Task {Filtering project content by the "Sequence accession number" criterion}

  It is possible to stop the loop by cancelling these tasks.

Sometimes a crash happens on disconnecting from the shared database.

Affected GUI tests, which failed by timeout:

• GUITest_common_scenarios_project_filtering_test_0001
• GUITest_common_scenarios_project_filtering_test_0002
• GUITest_common_scenarios_project_filtering_test_0003
• GUITest_common_scenarios_project_filtering_test_0004
• GUITest_common_scenarios_project_filtering_test_0005
• GUITest_common_scenarios_project_filtering_test_0006
• GUITest_common_scenarios_project_filtering_test_0007
  GUITest_common_scenarios_project_filtering_test_0011
  GUITest_common_scenarios_project_filtering_test_0013

On release 1.27 there are no any errors in the log, the 2 running tasks are present and can be cancelled.