Using layout panels in this context is not yet supported. Therefor, the fix
is to raise an exception instead of crashing.
The problem here is that layout panels need an a parent panel which
has a persistent state. The popover currently does not only has a very
short-lived panel currently that is freed directly after drawing.
Also see `ui_paneltype_draw_impl`.
It's likely that support for this can be added in the future but it's currently
unclear how many changes this would require.
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