Fixed MaterialTree: NullPointerException when deselectSelectedItem execute #378

Author: kevzlou7979

src/main/java/gwt/material/design/addins/client/tree/MaterialTree.java

@@ -98,7 +98,9 @@ protected void onLoad() {
                 }
             }
             MaterialTreeItem treeItem = event.getSelectedItem();
-            treeItem.addStyleName(AddinsCssName.SELECTED);
+            if (treeItem != null) {
+                treeItem.addStyleName(AddinsCssName.SELECTED);
+            }
             setSelectedItem(treeItem);
         }));
     }