This error comes when you try to add new datasource to the top of the query.
QueryBuildDataSource qbds;
qbds = query.addDataSource(tableNum(newTable)); // This is wrong
You should add new table to the Main datasource instead.
query.dataSourceTable(tableNum(newTable)).addDataSource(..) // This is correct
QueryBuildDataSource qbds;
qbds = query.addDataSource(tableNum(newTable)); // This is wrong
You should add new table to the Main datasource instead.
query.dataSourceTable(tableNum(newTable)).addDataSource(..) // This is correct