You are using an older version of AG Grid. To upgrade, please see the migration guide.
Full Error Text
Theming API and Legacy Themes are both used in the same page. A Theming API theme has been provided to the 'theme' grid option, but the file (ag-grid.css) is also included and will cause styling issues. Remove ag-grid.css from the page. See the migration guide: https://www.ag-grid.com/archive/33.0.4/javascript-data-grid/theming-migration/
To show this error text in the dev console, import the ValidationModule.