Bug 148626 [Regression]Can't fetch value from the filter of table/chart which inherites data from container
Steps: 1. create a table and bind a dataset in it 2. create another table/chart in the table detail/table group header 3. switch to the Filter tab in property editor 4. create a filter and select value in "<select value...>" combo 5. an error popuped up, "The expression value can't be retrieved"
chart/org.eclipse.birt.chart.reportitem.ui/src/org/eclipse/birt/chart/reportitem/ui/ReportDataServiceProvider.java
URL[] urls = new URL[count];
		for ( int i = 0; i < count; i++ )
		{
			File file = new File( classPathArray[i] );
			try
			{
				urls[i] = file.toURL( );
			}
			catch ( MalformedURLException e )
			{
				urls[i] = null;
			}
		}
		return new URLClassLoader( urls, parent );
	}
	
	
	/* (non-Javadoc)
	 * @see org.eclipse.birt.chart.ui.swt.interfaces.IDataServiceProvider#getDataType(java.lang.String)
	 */
	public DataType getDataType( String expression )
	{
		if ( expression == null || expression.trim( ).length( ) == 0 )
		{
			return null;
		}
		Iterator iterator = itemHandle.columnBindingsIterator( );
		while ( iterator.hasNext( ) )
		{
			ComputedColumnHandle cc = (ComputedColumnHandle) iterator.next( );