Don't break dashboard if widget is no longer available
If a PHP Class for a widget does no longer exist, the whole dashboard breaks.
An example would be:
( ! ) Fatal error: Class 'TYPO3\CMS\Dashboard\Widgets\AbstractDoughnutChartWidget' not found in htdocs/typo3conf/ext/styleguide/Classes/Widgets/Pages.php on line 32
This widget extends the AbstractDoughnutChartWidget, which no longer exists, as I've removed the cherry picked patch. I guess the same behaviour would happen once an extension is removed from the system, which provided a widget that was added to a dashboard.