Combobox Component Vaadin 7

Flow
1

Hi,
I am getting Invocation of method valueChange in VersionSelectorViewImpl$1 failed Caused by: java.lang.NullPointerException.
I traced the execution and the value I am passing for version in versionSelector.setValue(version) is a legitimate value (string).

This is how I create the component:
public class VersionSelectorViewImpl implements VersionSelectorView{

// Create a selection component;
private ComboBox versionSelector = new ComboBox();
private Listener listener;



public VersionSelectorViewImpl(){
	construct();
}

private void construct() {
	versionSelector.setVisible(false);
	versionSelector.setSizeUndefined();
    versionSelector.setImmediate(true);
    versionSelector.setNullSelectionAllowed(false);
    versionSelector.setTextInputAllowed(false);
    versionSelector.addStyleName("version-selector");
    //setup listener for the selected item
    versionSelector.addValueChangeListener(new Property.ValueChangeListener() {
       @Override
       public void valueChange(Property.ValueChangeEvent event) {
           if (listener != null) {
              listener.versionSelected((String) event.getProperty().getValue());
          }
      }
  });
}


@Override
public Component asVaadinComponent() {
	 return versionSelector;
}

@Override
public void setListener(Listener listener) {
	this.listener = listener;
	
}


@Override
public void setCurrentVersion(String version) {
	 if (versionSelector != null) {
            
            //select an item with the corresponding setValue() method
            versionSelector.setValue(version);
           
        }
	
}

@Override
public void setVisible(boolean visible) {
	versionSelector.setVisible(visible);
	
}

@Override
public boolean isVisible() {
	return versionSelector.isVisible();
}

@Override
/**
 * Adds items in the ComboBox for the selected page
 */
public void setAvailableVersions(List<GitVersionInfo> versions) {
	 if (versions != null && !versions.isEmpty()) {
		 versionSelector.removeAllItems();
            for (GitVersionInfo version : versions) {
               
                                
                //set caption mode for the combobox items
                //versionSelector.setItemCaptionMode(ItemCaptionMode.EXPLICIT_DEFAULTS_ID);
                //Add an item with a generated ID
                versionSelector.addItem(version.getVersionID());
               
                // Set item caption for this item explicitly
                String timeDifference = calculateTimeDiff(version.getVersionDate());
                versionSelector.setItemCaption(version.getVersionID(), version.getVersionName()+ "/ " + timeDifference
                			+ " By " + version.getVersionUser() + "/" + version.getVersionComment());
                	                                
            }
            versionSelector.setVisible(true);
        } else {
        	versionSelector.setVisible(false);
        }
    }
	``

Thanks in advance,
Ana