Actual Results:
When using TitlesMultiselectWidget it will cause a javascript error and an output where the widget does not load.
Uncaught Error: Unknown widget type: id: mw-input-wpTest, class: mw.widgets.TitlesMultiselectWidget at Object.OO.ui.Element.static.unsafeInfuse (load.php?lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|jquery.ui&skin=timeless&version=1wca1:162) at deserialize (load.php?lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|jquery.ui&skin=timeless&version=1wca1:163) at Object.OO.copy (<anonymous>:305:662) at Object.OO.copy (<anonymous>:306:192) at Object.OO.ui.Element.static.unsafeInfuse (load.php?lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|jquery.ui&skin=timeless&version=1wca1:162) at Object.OO.ui.Element.static.infuse (load.php?lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|jquery.ui&skin=timeless&version=1wca1:161) at Object.OO.ui.infuse (load.php?lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|jquery.ui&skin=timeless&version=1wca1:157) at HTMLDivElement.<anonymous> (<anonymous>:235:420) at Function.each (load.php?lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|jquery.ui&skin=timeless&version=1wca1:5) at jQuery.fn.init.each (load.php?lang=en&modules=jquery%2Coojs-ui-core%2Coojs-ui-widgets|jquery.ui&skin=timeless&version=1wca1:3)
Expected Results:
For the widget to load properly and not display that error.
Downstream task: https://phabricator.miraheze.org/T6877