Fixed input to ensureAvailabilty (#12208)

client/src/main/java/com/vaadin/client/ui/treegrid/TreeGridConnector.java

                             .getVisibleRowRange();
                     getDataSource().ensureAvailability(
                             visibleRowRange.getStart(),
-                            visibleRowRange.getEnd());
+                            visibleRowRange.length());
                 }
                 checkExpand();
             }
                             .getVisibleRowRange();
                     getDataSource().ensureAvailability(
                             visibleRowRange.getStart(),
-                            visibleRowRange.getEnd());
+                            visibleRowRange.length());
                 }
                 checkExpand();
             }