Bug 136553 [regression]Preview error when set invalid group interval.
Description: Preview error when set invalid group interval. Steps to reproduce: 1. Insert a table 2. Add a group on number column, set interval to Year,1 3. Preview 4. Edit group on string column set interval to Prefix,1 5. Preview Expected result: Error msg about invalid group interval. Actual result: Error happened while running the report
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/data/dte/DteDataEngine.java
ex.printStackTrace( );
		}
	}
	protected void doPrepareQuery( Report report, Map appContext )
	{
		
		List queries = report.getQueries( );
		for ( int i = 0; i < queries.size( ); i++ )
		{
			IQueryDefinition query = (IQueryDefinition) queries.get( i );
			try
			{
				IPreparedQuery preparedQuery = dteEngine.prepare( query,
						appContext );
				queryMap.put( query, preparedQuery );
			}
			catch ( BirtException e )
			{
				logger.log( Level.SEVERE, e.getMessage( ), e );
				context.addException( e );
			}
		} 
	}
	protected IResultSet doExecuteQuery( IBaseQueryDefinition query )
	{
		assert query instanceof IQueryDefinition;
		IPreparedQuery pQuery = (IPreparedQuery) queryMap.get( query );
		if ( pQuery == null )
		{
			return null;