Fixed dropdzone unload

Author: kevzlou7979

src/main/java/gwt/material/design/addins/client/fileuploader/MaterialFileUploader.java

@@ -127,6 +127,9 @@ protected void build() {
         options = getDefaultOptions();
     }
 
+    @Override
+    protected void onUnload() {}
+
     @Override
     protected void initialize() {
         if (getWidgetCount() > 1) {