Bug 153387 Database item can't be dragged and dropped onto editor field
Steps to reproduce: 1.Start up BIRT in Mac OS X; 2.File->New->Project; 3.File->New->Report Design; 4.In Data Explorer, create a New Data Source, and choose "Classic Models Inc. Sample DataBase", press "Next" and then press "finish"; 5.In Data Explorer, create a New Data Set, press "Next"; 6.In the popup window, expand the "CLASSICMODELS", select and drag a item onto the editor field at right side.
data/org.eclipse.birt.report.data.oda.jdbc.ui/src/org/eclipse/birt/report/data/oda/jdbc/ui/editors/SQLDataSetEditorPage.java
* 
	 * @return
	 */
	private DataSetDesign getDataSetDesign( )
	{
		if ( dataSetDesign == null )
			dataSetDesign = this.getInitializationDesign( );
		return dataSetDesign;
	}
	
	/**
	 * populate shema list if the schema is supported
	 * 
	 */
	protected void populateSchemaList( )
	{
		if ( rootNode != null )
		{
			removeTreeItem( rootNode );
		}
		getAvailableSchema( );
		
		if ( schemaCombo.getItemCount( ) < 1 )
		{
			schemaCombo.add( JdbcPlugin.getResourceString( "tablepage.text.All" ) );
			schemaCombo.select( 0 );
			if ( schemaList != null )
			{
				Iterator it = schemaList.iterator( );
				while ( it.hasNext( ) )