AbstractCellProvider Type Not Found

I am hoping to display a list of editable nodes underneath a cousin node in its editor.  Note that the editable nodes are not descendants but cousins of the target node.  If this is not possible, please let me know.

To do this, I have added a "custom cell" to the editor for the target node, which will walk the AST to find the cousins and create edit cells for each cousin.  This custom cell defines a cell provider that returns an instance of AbstractCellProvider.  However, when I attempt to create a local variable of type "AbstractCellProvider", I find that this type is not included in the completion menu.

I have verified that my "editor" model uses the language jetbrains.mps.lang.editor.  The definition of the cell provider contains a reference to AbstractCellProvider as the return value, and that type reference seems to be OK, so I do not know why I cannot declare a local variable with the same type.

I have attached a screenshot that shows what I am trying to do.