Bug 141950 Selection of select value in data set filter is error
Details: Selection of select value in data set filter is error Step: 1.New a report design. 2.Create a sample datasource and dataset. 3.Switch to filter. 4.Select a value in expression. 5.Drop the list of the value1 and select <select value...> Actual result: No value is appear and pop up an error. It said that "An expression has not been set"
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/data/ui/util/DataSetProvider.java
{
	private static DataSetProvider instance = null;
	
	private transient Hashtable htColumns = new Hashtable( 10 );
	private static Hashtable htDataSourceExtensions = new Hashtable( 10 );
	private transient Hashtable sessionTable = new Hashtable( 10 );
	
	private static final char RENAME_SEPARATOR = '_';
	private static String UNNAME_PREFIX = "UNNAMED"; 
	/**
	 * @return
	 */
	private static DataSetProvider newInstance( )
	{
		return new DataSetProvider( );
	}
	/**
	 * 
	 * @return
	 */
	public static DataSetProvider getCurrentInstance( )
	{
		if ( instance == null )
			instance = newInstance( );