Don't attempt to scroll NavigationTable until it is attached

Computing the scroll information for an unattached table is
useless. The UI state is not known and may be updated at a
later point in time.

This skips a bit of unnecessary work when a table is built
in the background, like FileTable does for ChangeScreen2.

Change-Id: I2892ae36a80f5158e30e019ebf9d6e865e348e93

gerrit-gwtui/src/main/java/com/google/gerrit/client/ui/NavigationTable.java

@@ -200,7 +200,7 @@ public abstract class NavigationTable<RowItem> extends FancyFlexTable<RowItem> {
       table.setWidget(newRow, C_ARROW, pointer);
       final Element tr = DOM.getParent(fmt.getElement(newRow, C_ARROW));
       UIObject.setStyleName(tr, Gerrit.RESOURCES.css().activeRow(), true);
-      if (scroll) {
+      if (scroll && isAttached()) {
         scrollIntoView(tr);
       }
     } else if (clear) {