preview stage won't have the metadata populated, so protect NPE

main/src/com/google/refine/commands/row/GetRowsCommand.java

@@ -180,7 +180,9 @@ public class GetRowsCommand extends Command {
             }
             
             // metadata refresh for row mode and record mode
+	    if (project.getMetadata() != null) {
             	project.getMetadata().setRowCount(project.rows.size());
+	    }
         } catch (Exception e) {
             respondException(response, e);
         }