For the *very* minimal vala test code, clone 
https://github.com/baedert/gtk-sw-bug.

The important parts are probably:

== main.vala ==
[GtkTemplate (ui = "/org/baedert/test/foo-scrolled.ui")]
class FooScrolled : Gtk.ScrolledWindow {}

void main (string[] args) {
  Gtk.init (ref args);
  var s = new FooScrolled ();
}


== foo-scrolled.ui ==
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="FooScrolled" parent="GtkScrolledWindow">
    <property name="visible">True</property>
  </template>
</interface>


Removing the visible property from foo-scrolled.ui makes everything work
as expected.
This is not reproducible with 3.12.2, only with current master.

backtrace:

+ Trace 233642