Clicking WebExtensions page action produces a critical
** (epiphany:2): CRITICAL : 17:10:23.063: void webkit_web_view_run_javascript_in_world(WebKitWebView, const gchar, const char*, GCancellable*, GAsyncReadyCallback, gpointer): assertion 'WEBKIT_IS_WEB_VIEW(webView)' failed
And does nothing otherwise. It might be a symptom of a bigger issue too: the icons are also empty here.