"Display | Synth" menu item check not in sync with synth display

Context: commit e65d6b8b (2013-05-10)

Steps:
1) Open or create a score
2) With "Display | Synthesizer" show the synth window
3) Close the synth window with its own close icon ('X' right top corner)
4) Look at the "Display" menu

Result: the check on the "Synthesizer" item is still there.

Selecting the item does nothing (the program tries to hide the synth window, which is already hidden).

Trying again the check is gone and selecting the item displays the synth again.