"Setup > Manage instruments" menu item disabled after opening it once

[juh2600]

Observed in Release build of ecc525e.

Expected behavior: When I open the "Manage instruments" dialog from the Setup menu, and then close the dialog, I expect to be able to re-open the same dialog through the same steps.

Observed behavior: After opening the "Manage instruments" dialog, the menu item is disabled, and does not enable after the dialog has been closed, making it apparently impossible to reopen.

Steps to reproduce:

1. Launch ngscopeclient
2. In the menu bar, open Setup > Manage instruments
3. [optional] Click Setup again and observe that "Manage instruments" is now disabled
4. Close the dialog
5. Click Setup again and observe that "Manage instruments" is still disabled