minor: style fix (use the variable as close to their declaration as possible)

SVN: 20068

screening/source/java/ch/systemsx/cisd/openbis/plugin/screening/client/web/client/application/detailviewers/ChannelChooserPanel.java

@@ -253,10 +253,9 @@ public class ChannelChooserPanel extends LayoutContainer
 
         String selectedComboValue = channelsComboBox.getSimpleValue();
         boolean showCheckBoxGroup = ScreeningConstants.MERGED_CHANNELS.equals(selectedComboValue);
-
-        ensureAtLeastOneCheckboxChecked();
         channelsCheckBoxGroup.setVisible(showCheckBoxGroup);
 
+        ensureAtLeastOneCheckboxChecked();
         notifySelectionListeners(selection);
     }