Bug 124600 delete Grid Column causes NPE thrown out in error log
steps to reproduce: - insert a 3*3 grid - delete grid columns one by one expected behavior: - these operations should completed without any error. actual behavior: - after i deleted the 3rd column, NPE was thrown out in error log.
UI/org.eclipse.birt.report.designer.core/src/org/eclipse/birt/report/designer/util/DNDUtil.java
*            source to clone
	 * @return copy of source
	 */
	public static Object cloneSource( Object source )
	{
		source = unwrapToModel( source );
		if ( source instanceof Object[] )
		{
			Object[] array = (Object[]) source;
			ArrayList list = new ArrayList( );
			for ( int i = 0; i < array.length; i++ )
			{
				if ( array[i] instanceof ColumnHandle )
				{
					list.add( cloneSource( array[i] ) );
				}
			}
			if ( !list.isEmpty( ) )
			{
				return list.toArray( );
			}
			for ( int i = 0; i < array.length; i++ )
			{
				
				if ( !checkContainerExists( array[i], array ) )
				{
					list.add( cloneSource( array[i] ) );
				}
			}
			return list.toArray( );