exception for subquery expression without source tables

[reata]

SELECT (SELECT col1 + col2 AS result) AS sum_result
FROM bar

$ sqllineage -f test.sql 
Traceback (most recent call last):
  File "/home/hujunwei/.pyenv/versions/3.8.6/bin/sqllineage", line 8, in <module>
    sys.exit(main())
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/cli.py", line 101, in main
    runner.print_table_lineage()
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/runner.py", line 168, in print_table_lineage
    print(str(self))
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/runner.py", line 22, in wrapper
    self._eval()
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/runner.py", line 177, in _eval
    self._stmt_holders = [analyzer.analyze(stmt) for stmt in self._stmt]
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/runner.py", line 177, in <listcomp>
    self._stmt_holders = [analyzer.analyze(stmt) for stmt in self._stmt]
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/core/parser/sqlparse/analyzer.py", line 62, in analyze
    self._extract_from_dml(stmt, AnalyzerContext())
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/core/parser/sqlparse/analyzer.py", line 221, in _extract_from_dml
    next_handler.handle(sub_token, holder)
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/core/parser/sqlparse/handlers/base.py", line 37, in handle
    self._handle(token, holder)
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/core/parser/sqlparse/handlers/source.py", line 65, in _handle
    self._handle_column(token)
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/core/parser/sqlparse/handlers/source.py", line 112, in _handle_column
    self.columns.append(SqlParseColumn.of(token))
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/core/parser/sqlparse/models.py", line 76, in of
    source_columns = SqlParseColumn._extract_source_columns(expr)
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/core/parser/sqlparse/models.py", line 118, in _extract_source_columns
    source_columns = [
  File "/home/hujunwei/.pyenv/versions/3.8.6/lib/python3.8/site-packages/sqllineage/core/parser/sqlparse/models.py", line 119, in <listcomp>
    ColumnQualifierTuple(src_col.raw_name, src_col.parent.raw_name)
AttributeError: 'NoneType' object has no attribute 'raw_name'