**How to reproduce**
Clickhouse version 21.2-21.3

```
SELECT x
FROM
(
    SELECT 1 AS x
    UNION DISTINCT
    SELECT 1
)
LIMIT 1

Query id: b82bbb8d-dea9-4f1c-9a2f-5b0e9b6c2422


0 rows in set. Elapsed: 0.002 sec.

Received exception from server (version 21.2.3):
Code: 49. DB::Exception: Received from localhost:9000. DB::Exception: Bad cast from type DB::ASTSelectWithUnionQuery to DB::ASTSelectQuery.
```


{
    Pipe pipe(std::make_shared<NullSource>(source_header));
    if (query_info.prewhere_info)
    {
        if (query_info.prewhere_info->alias_actions)
        {
            pipe.addSimpleTransform([&](const Block & header)
            {
                return std::make_shared<ExpressionTransform>(header, query_info.prewhere_info->alias_actions);
            });
        }
        pipe.addSimpleTransform([&](const Block & header)
        {
            return std::make_shared<FilterTransform>(
                header,
                query_info.prewhere_info->prewhere_actions,
                query_info.prewhere_info->prewhere_column_name,
                query_info.prewhere_info->remove_prewhere_column);