[Crash, wd, debug] Close wd tab with Discard button on some debug stage.

1. Close the project if it is open (File->Close project).
2. Enable the debug mode:
   1. "Preferences..."- >"Workflow Designer"->"Runtime settings".
   2. Check "Enable debugger".
   3. Click "OK" to confirm.
3. Open the WD ("Run or Create Workflow" on the "Start Page").
   The "Choose Output Directory" dialog may appear. Set any directory and click "OK".
   Finally the WD window appeared.
4. Open "Samples"- >"Alignment"->"Align sequences with MUSCLE".
   The "Align Sequences with MUSCLE Wizard" appears.
5. Set "COI.aln" from data/samples/CLUSTALW as "Input MSA(s)":
6. Click "Next >" and "Apply" to confirm and return to the WD window.
7. Set a breakpoint on the "Align with MUSCLE" element:
   1. Select the "Align with MUSCLE" element on the Scene by clicking on it.
   2. Press Ctrl+B (⌘+B) (Break at element).
      The "Align with MUSCLE" element has a red frame.
8. Press Ctrl+R (⌘+R) (Run workflow).
9. The breakpoint was triggered. Open the message table:
   1. Click on the one  of two connections: between the "Align with MUSCLE" and "Write alignment" elements or between "Read alignment" and "Align with MUSCLE"
   2. An empty table "Messages from 'Align with MUSCLE' to 'Write alignment'" appears.
10. Close workflow tab by pressing shutdown button "х" in the right corner.
11.  Click "Discard" button in the dialog with message: "The workflow has been modified.
    Do you want to save changes? "

Expected: workflow is closed.
Actual: Crash.

Note, there will be no crash, if we skip step 9 with opening an empty table "Messages from 'Align with MUSCLE' to 'Write alignment'".

Crash report: 26048