Fix NavigationTable bug allowing selection of invalid rows

Clicking on a row should not cause selection. Valid rows are
identified by having a non-null row item available, so test
for the row item before moving the pointer onto the row.

Change-Id: Iffe1a4bb57db53ca3bb31ca1bd43c00ee6e70449

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

@@ -189,6 +189,10 @@ public abstract class NavigationTable<RowItem> extends FancyFlexTable<RowItem> {
   }
 
   protected void movePointerTo(final int newRow, final boolean scroll) {
+    if (getRowItem(newRow) == null) {
+      return;
+    }
+
     final CellFormatter fmt = table.getCellFormatter();
     final boolean clear = 0 <= currentRow && currentRow < table.getRowCount();
     if (clear) {