Public URL for a dashboard that contains a text widget throws 500 server error

[madalincm]

Issue Summary:

Build identifier: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:55.0) Gecko/20100101 Firefox/55.0

Steps to Reproduce:

1. Open a dashboard that contains a text widget.
2. Generate a public url and try to open that url.

Expected results:

The url can be loaded, the dashboard is displayed.

Actual results:

500 Error displayed in the console, the dashboard can't be accessed.

Notes/Issues:

Verified in FF55(Win7). Issue can be reproduced only in STMO stage and STMO prod

Example of a dashboard that contains a text widget: https://pipeline-sql.stage.mozaws.net/dashboard/textwidgetservererrorinpublicurl

Public URL that returns the server error: https://pipeline-sql.stage.mozaws.net/public/dashboards/fOO9BUW3svpUGgbqI59Honzgv5XjfufvsTllhqma?org_slug=default

[washort]

Looks like this is a result of the changes for #166: https://sentry.prod.mozaws.net/operations/data-stmo-stage/issues/642969/events/11758337/

[madalincm]

Verified fixed in FF55(Win7) in stmo-stage.