Disappearing plate

Fix the disappearing issue described in https://trac.openmicroscopy.org.uk/ome/ticket/10510

To test:
 * Browse a plate, click on a well sample (central panel). The grid should be displayed.
 * Click on the experimenter (For example). The grid should be removed from the display.
 * Select the plate again.
 * Click the Refresh button. The grid should be removed from the display.