Bug 146159 [Regression]Changing font operation doesn't work other than the first time
Description: The first time changing operation can work well. However, Change it again, it won't work. Steps: 1.Insert lable/text to layout or master page 2.Change the font from serif to cursive, it works well 3.Change the font again Result: The action doesn't work Additional infor: build number:200606080632
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/internal/ui/dialogs/FontPreferencePage.java
Group group = new Group(getFieldEditorParent(), SWT.SHADOW_OUT);
		group.setText(Messages.getString("FontPreferencePage.text.Preview")); 
		GridData gd = new GridData(GridData.FILL_HORIZONTAL);
		gd.widthHint = 400;
		gd.heightHint = 100;
		gd.horizontalSpan = 4;
		group.setLayoutData(gd);
		group.setLayout(new GridLayout());
		sample = new PreviewLabel(group, SWT.NONE);
		sample.setText(Messages
				.getString("FontPreferencePage.text.PreviewContent")); 
		sample.setLayoutData(new GridData(GridData.FILL_BOTH));
		UIUtil.bindHelp( getFieldEditorParent( ).getParent( ),IHelpContextIds.STYLE_BUILDER_FONT_ID ); 
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.preference.FieldEditorPreferencePage#createContents(org.eclipse.swt.widgets.Composite)
	 */
	protected Control createContents(Composite parent) {
		Control ct = super.createContents(parent);
		updatePreview();
		return ct;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.preference.FieldEditorPreferencePage#propertyChange(org.eclipse.jface.util.PropertyChangeEvent)
	 */
	public void propertyChange(PropertyChangeEvent event) {