title: Airflow - Chart Query failing
body: <p>I am new to Airflow and I am trying to create charts for my DAG but I keep getting the following error:</p>
  
  <pre><code>SQL execution failed. Details: 'NoneType' object has no attribute 'get_pandas_df'
  </code></pre>
  
  <p>My query looks like: </p>
  
  <pre><code>SELECT dag_id, execution_date, count(*) as ccount FROM task_instance GROUP BY dag_id, execution_date
  </code></pre>