[GitHub] [airflow] drexpp opened a new pull request #6000: [AIRFLOW-5393] UI crashes in the Ad Hoc Query menu (PULL REQUEST FIX)

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drexpp opened a new pull request #6000: [AIRFLOW-5393] UI crashes in the Ad Hoc Query menu (PULL REQUEST FIX)
URL: https://github.com/apache/airflow/pull/6000
 
 
   Make sure you have checked _all_ steps below.
   
   ### Jira
   
   - [X] My PR addresses the following [Airflow Jira](https://issues.apache.org/jira/browse/AIRFLOW/) issues and references them in the PR title. For example, "\[AIRFLOW-XXX\] My Airflow PR"
     - https://issues.apache.org/jira/browse/AIRFLOW-5393
     - In case you are fixing a typo in the documentation you can prepend your commit with \[AIRFLOW-XXX\], code changes always need a Jira issue.
     - In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal ([AIP](https://cwiki.apache.org/confluence/display/AIRFLOW/Airflow+Improvements+Proposals)).
     - In case you are adding a dependency, check if the license complies with the [ASF 3rd Party License Policy](https://www.apache.org/legal/resolved.html#category-x).
   
   ### Description
   
   - [X] Here are some details about my PR, including screenshots of any UI changes:
   
   There was a crash in the Airflow Ad Hoc Query menu when you hit the ".csv" button to obtain a csv file of the query if the query is empty. This happened due to a "**_df variable referenced before assigment_**", the "**_df_**" variable is catched inside a try / except block and "**_df_**" is never assigned, instead there is an "**_error_**" variable which can be used to detect when the query was empty or the connection could not be stablished.
   
   In the screenshot it fails in line 2295 and the "**_error_**" variable will be True.
   
   In line 2317 you can see the proposed change adding "**and not error**"
   
   ### Tests
   
   - [X ] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason:
   
     - I didn't know exactly how to unit test this, if you have any advice I will do a test for it. Other than that, I did test checking that the behaviour was as expected:
       
       - [] Querying with blank text and unset connection works with an error message
       -  Querying with blank text and set connection works with an error message
       - Querying with text and set connection works returning the .csv file
       - Querying with text and unset connection works returning an error message
   
   
   ### Commits
   
   - [X ] My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "[How to write a good git commit message](http://chris.beams.io/posts/git-commit/)":
     1. Subject is separated from body by a blank line
     1. Subject is limited to 50 characters (not including Jira issue reference)
     1. Subject does not end with a period
     1. Subject uses the imperative mood ("add", not "adding")
     1. Body wraps at 72 characters
     1. Body explains "what" and "why", not "how"
   
   ### Documentation
   
   - [ ] In case of new functionality, my PR adds documentation that describes how to use it.
     - All the public functions and the classes in the PR contain docstrings that explain what it does
     - If you implement backwards incompatible changes, please leave a note in the [Updating.md](https://github.com/apache/airflow/blob/master/UPDATING.md) so we can assign it to a appropriate release
   
   ### Code Quality
   
   - [ ] Passes `flake8`
   

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