Some (most) Silk UI widgets are composed of other widgets you can drag onto its Placeholders. It's hard to understand what the platform expects, and wether any widget will work (this is not always the case -- some odd widgets break the enclosing widget, as far as I could tell).
Why can't you specify which widgets are allowed inside a Placeholder, when creating Placeholders? This way, if you select that Placeholder after its widget has been dragged onto the canvas, OutSystems 10 could grey-out widgets that are not allowed to be dragged inside it, on the left-hand-side widget list. It could also refuse to let you drop such unallowed widgets onto the Placeholder.
There should be some way to defeat this so that your own (new) widgets may be added to any Placeholder's list. Maybe this could be done by defining equivalence of new widgets to existing widgets, in terms of what is allowed in existing Placeholders.