Using 3.1.2 RC1 plus latest commits from Github.

Create a simple element in article builder .

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Assign to the category of the article and you will see the widget.

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Do the above but instead of an element, add a sublayout.

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The result shows no widgets available, error reporting on maximum shows the following.

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Each error is for each element in the sublayout, i.e the sublayout has three elements.

If you add an element and a sublayout, only the element shows in the widgets.

NOTE:- The sublayout does show up in the article, just the widget is the issue.