Web interface hangs after query-submission error

On the query page, getting an error seems to jam up the web application.

Example 1:
1. Go to query page.
2. Click on Submit button (with empty query, type defaulted to "SQL").
3. (Get "HTTP ERROR 500 \n Problem accessing /query. Reason: \n Request failed." error.)
4. Click on browser Back button.
5. (Get query page.)
6. Click on Submit button.
7. Web application hangs--browser indicates it's waiting. (In current case, Mozilla SeaMonkey says "Waiting for localhost..." and "Stop" icon is enabled.)

Example 2:
1. Go to query page.
2. Copy sample query ("SELECT * FROM cp.`employee.json` LIMIT 20") and paste into query box.
3. Click Submit button.
4. (Get tabular results page.)
5. Click on browser Back button.
6. (Get query page, with query still there.)
7. Edit query, adding apparent syntax error (e.g., append ";" to end).
8. Click Submit button.
9. (Get "HTTP ERROR 500" page (rather than good "syntax error" message).)
10. Click on browser Back button.
11. (Get query page, with (bad) query still there.)
12. Fix query (e.g., remove bad appended ";" from end).
13. Click Submit button.
14. Web application hangs--browser indicates it's waiting.