Firefox crash at gtk_css_selector_tree_collect_change()

This is a follow up from https://bugzilla.mozilla.org/show_bug.cgi?id=1619388 Backtrace of the crash: https://crash-stats.mozilla.org/report/index/b25ad6b5-8710-455b-8693-6dcf80200302

It's a null pointer crash at gtk_css_selector_tree_collect_change() when new button is created by gtk_link_button_new().