Fix the clear issue in MaterialListValueBox.

Author: BenDol

gwt-material/src/main/java/gwt/material/design/client/ui/MaterialListValueBox.java

@@ -144,6 +144,7 @@ public OptionElement getOptionElement(int index) {
      */
     @Override
     public void clear() {
+        values.clear();
         listBox.clear();
         if (initialized) {
             // reinitialize
@@ -237,7 +238,6 @@ public void onValueChange(ValueChangeEvent<T> event) {
 
     @Override
     public void setAcceptableValues(Collection<T> values) {
-        this.values.clear();
         clear();
 
         for(T value : values) {