The steps in #132900 would lead to a failed assert and some warning prints:
"Warning: Asset loading is unfinished". This is because asset libraries get
cleared more strictly and correctly since a859ed1130, but only once the menu
was shown it would get reloaded.
Instead make sure the asset library loading gets triggered whenever a button to
popup a brush asset shelf is present. This makes sense because in this context
you typically want an asset library available.
c07b253265